TL;DR: There's a LOT of advice that isn't "write every day" even about how frequently one OUGHT TO be writing, and you certainly should write only as much as you derive meaning and satisfaction from, but the reason you're likely to hear this one a lot is because people who are unhappy with their lack of writing career constantly ask working professionals how they "made it," like there's a trick to doing something professionally. Read a lot, write as much as you can, trust the process (particularly including peer review), be deliberate with your writing (and reading), and check in with some folks who've gone before you so that you're not spinning your wheels quite so much.
The devil's due:
Even as an explicit question about advice that is NOT write every day, it is worth taking a moment to explain why this is such common, such good, and such conventional-wisdom-esque advice among working writers. Basically, I can't give you all the "other" advice without a massive, thirty-foot-tall disclaimer with flaming letters that the best damn thing you could possibly do if you want to be a novelist or some kind of working creative writer is to set aside as much time a day as you possibly can at the same TIME every day and sit down and write.
1) Because it works. There are few skills at which one can improve so quickly and predictably as writing, and there are really NO paths to prose improvement that do not involve consistent work. Creativity is like a muscle. With the exception of some folks with neurological limitations, if you set aside the same time each day to do something creative, you WILL get better at it in an entirely predictable way––starting to have ideas about 10-15 minutes before your "session" begins. You can kind of "aim" it and it sort of obeys your command, but it's not entirely under your control. (This is why I sometimes call it The Force. "You mean, it controls your actions?" "Partially. But it also obeys your command.")
I could wallpaper a room with all the testimonials I have gotten since I started blogging that writing every day turns out to work amazingly well, that people found their muse, finished their shit, and were able to write consistently when they sat down. (Though, admittedly, if I kept it in 12-point font, it would have to be a very small room.)
2) Because it's metonymy. Look, if you don't tell the writers' cabal of my transgression, I'll let you in on a little secret. You don't have to write EVERY day. "Write every day" is just an easier slogan than "Write five or six times a week unless you're sick, but it's really good to do a little something on those off days if you can, and....." Well, you get the idea.
Most of the writers with careers that people envy write every day, but you can make a living doing six days a week. Maybe even five (though by conventional successful-writer wisdom, that's REALLY pushing it). You can spend a couple of days a week writing for a couple of hours instead of five or six. (This is what I do. I have weekends.) You can take a couple of hours writing three really long emails and call it a day. You can be distracted by the news, write six hefty Facebook posts, and then give up on doing something on your novel or blog. (It was still writing even if you were distracted.) What you're going for is the practice; it doesn't have to be a five-hour session on your work in progress. But also you don't want to lose that mental connection you have between ideas and the words that bring them to life, and like anything we practice at constantly where we are using a skill to turn our ideas into an expression other people can experience (say, like a musical instrument), you'll get rusty faster than you think. That's quite a mouthful; "Write every day" is easier to remember.
3) Because no one ever asks working writers how to be contented hobbyists. What working creative writers get asked is how the questioner can also be a working creative writer. Our success gets "probed" by people wondering about agents or publishing nepotism or our social media marketing strategies like there is a secret. Yes, there are influences that are unearned advantages of birth and cannot be controlled like being white, raised middle class or higher, having formally educated parents, being cishet, being male, and being from an anglophone nation (the last really only because of the sheer amount of publishing that comes out of New York). There are a few things that are like "force multipliers" like having social media outreach, nepotistic connections in publishing, or some entirely-unrelated-to-writing fame, but no one ever EVER got there without working outrageously hard and probably pretty close to daily.
Writers actually have a very "Do as thou wilt!" approach to other people's writing. You do you. You decide your own level of involvement. If you don't want to write every day....don't. I'm very clear that creative writing is not a path to riches or fame for 99.999% of those who love it. At best it is a long and arduous path to a very modest but fulfilling living where you will be tempted by the kinds of writing that pay better money (like technical writing, ghost writing, and even content writing). You can ARGUE with the fact that we writers have consistently noticed that every one of us who's crested the more-than-a-cell-phone-bill plateau or "made it" in some sense that the world considers meaningful tends to write daily or almost so, but it's not going to make it UN-true. Our advice is descriptive and empirical––we're not, like, holding back on folks until they haze themselves with daily writing. (And those that do treat this advice in this way are probably being ableist.) The fact is, most writers who make a tidy living (and particularly the ones that make a splooshy one) are the folks who are out there fiddling with their schedules, trying to find and justify MORE time writing, not less. But hey, you know, maybe getting mad might work THIS time.
The more you think of your brain as akin to a musical instrument, taking your ideas and emotions and converting them into a form others can appreciate, the more quickly you will realize that it is a skill that will atrophy with disuse, that you need lots of practice to be proficient, more to be "good," that being a hobbyist is okay if it makes you happy, but that being exceptional or "making it" will take constant training like most folks wouldn't believe.OTHER ADVICE
Remember, this isn't advice that's exclusive to people who can't write every day. It's just the other Very Important Advice™ that will create working writers. So if you can get to the page every day and ALSO do these things, you will advance even faster.
Write as much as you can: Okay, you can't, won't, or don't want to write every day. Fine. Do it as much as you can. Come close. You don't get better at anything by NOT doing it. If you want to get better at writing, write MORE. Write five days. Write six. Write as much as possible on the weekends but at least a fat paragraph during your lunch break three days a week during lunch. Whatever, just get as close as possible.
Read (or keep reading): A lot of writers stop reading. Like they kind of figure they read all the books they'll ever need early in their life and now it's time to just do the writing part. Don't do that. Trying to only write is like trying to only breathe OUT.
Occasionally read things you wouldn't normally: Tough books. Nonfiction. Western canon lit (if that's not your normal jam). A Pushcart anthology. A genre you don't usually dig. Once in a while take a stroll on a new path and see some new sights. You might learn a few things and get some WONDERFUL ideas.
Think about writing: Let me be honest with you. I hate this advice. Even though I have to grudgingly give it a half nod. I hate this advice because it has fueled so many fucking "Why don't I have a book deal yet?" entitled a-holes who tell you in that supercilious way that they don't NEED to write every day because they THINK about writing. (For some reason, I always imagine them taking a drag of a cigarette right between those two clauses.) And every last one of them was exactly the sort who was turning in that same retooled vignette in their capstone classes that they showed up with and workshopped on their first class of the program. This is just way too many pretentious wankers' "out" when it comes to applying their ass to the chair and doing some goddamn work. And I just fucking HATE that it might be tempting sincere and dedicated writers into losing a valuable habit. So if you can't write, think about writing. If you have a choice, though, pick the actual writing.
Also, this is not "I had a passing thought about my writing earlier today, so now I'm good." You want to actually spend 10-15 minutes considering word choices and elements of craft. Consider a character arc. Think about how exactly your setting could subtly reinforce your theme. Think about how to have emotional and personal stakes in your climax instead of just external ones.
Figure out EXACTLY why you like writing that you like: One of the reasons literature majors and creative writing majors spend about 90% of their time in the exact same classes is because the "close reading" of literature and the "how did the author make me feel this way" of creative writing are basically the same skill set––you get down into the guts of the sentence structure and specific word choice and see what made that meaning happen.
For a casual reader, it's fine to just read something and sigh wistfully. (Such beauty. Much prose. Wow!) Who amongst us hasn't pressed Victorian literature to their chest in desperate wanting? Well, actually I haven't but whatevawhoodles. However, to read "as a writer" means to pause when a passage takes your breath away, take a moment to look at exactly what moved you, and THEN ASK HOW? How is it doing what it's doing? Is it the language? If so, which specific words? Is it the sound it makes in your head? Is it the imagery? Is it the sentence construction? Or maybe the way long and short sentences weave together? Consciously notice what is going on. Unlock its secrets. Let that author teach you their tricks. Be the ready student, and the master that is that writer will reach across space and maybe even time and give you your very own private writing tutoring session. Read consciously.
Practice outside your comfort zone, but also practice writing that plays to your strengths: I love writing dialogue, and really hate trying to write about FEELINGS. So I often pause when I read good descriptions of feelings (above) and pay attention to that. I try to emulate it in prompts or when I'm writing on some draft.
However, when I'm writing for publication (especially a stretch goal publication and not a "safe" publication), I TEND to focus more on dialogue because I want to go where I'm strong. Consider some of the writing you do like practicing for a sport. If you suck at speed but are super good at endurance, you definitely want practice sessions to include speed drills so you work on that weakness and get better. However, in a competition with your crosstown rivals, you'll want to play to your endurance as much as you can and avoid situations requiring raw speed.
Start wherever (beginning or maybe not): Perhaps the weirdest thing about starting writers is they know but still refuse to accept that they're absolutely NOT going sit down and write their magnum opus book from beginning to end and then just go "clean up the grammar."
They know it, but they still don't....GROK it. They still insist on a contiguous experience and have the hardest time making cuts. It's okay to sit down and write the ONE scene you keep thinking about, even if it's near the end or even if it's just floating around and you're not sure when it will fit in. Just get it out. Perhaps it's future fodder, but maybe it's just practice. But the likelihood is as you start to get THAT scene out, that fucking loop in your head will stop, and suddenly you'll be thinking of ANOTHER scene. By the time you have finished writing scene 4, scene 13, and scene 22, you've probably thought of scene 7, 3, and 12. Then you can work backwards, sideways, upside down, or whatever timey wimey way you want.
Writing is a recursive thought process because it is literally impossible for you to write faster than you think. You will have ideas as you write, and some of them will be really good.
Routine!: Try to develop a daily routine in as much as that is possible for you, even if (or perhaps especially if) that routine involves a lot of rest and relaxation. It might be counterintuitive at first, but the more sort of...BORING your outside life is, the more your creative life tends to flourish. That doesn't mean you can't go on a vacation or something (though maybe you still try to wake up and do a half an hour every morning except for the day you're actually GOING to Disneyland). It means you embrace as much routine as you can. If you can come to the page at the same time every day, it's going to turn your creativity up to eleven. That's just the way our brains work. There are options for those who simply don't have the life that fosters routine, but getting as close as possible to one is the better choice.
Treat yourself well: We treat our brains like they're these psychic entities that live on other planes of existence that can only be reached by astral projection from the psi-vortexes within our skulls but our brains are right there with us not getting enough sleep, hurting from stress, and feeling kind of overloaded after that triple cheeseburger with greasy fries and a shake. Exercise a little (if you can). Eat decently (if you can). Drink enough water. Take your meds (if you can). Your brain is an organ. It's pretty awesome, but it has never NOT been a part of your body.
Trust the process––no, REALLY: This one might be the hardest for starting writers. Half the reason they sit frozen at their opening sentence is because somewhere inside they don't actually believe that they'll end up changing everything. They want to nail it on the first attempt.
You're going to have to write many drafts. You're going to need peer review. You're going need to change some stuff. You're not the chosen one who won't need to rewrite your book and make huge changes. You're not the special snowflake who won't get some harsh feedback. You're not the messiah of writing who won't have to practice for years. The process is long, messy, and sometimes really painful but the less you trust it, ironically, the more it gets longer, messier, and even MORE painful.
Do peer review: A special shout out to the part of the process people tend to trust the least. It's gonna sting. You won't like it at first. You're brilliant and why can't they see that? Seriously, they didn't notice that thing you did? Who are these clowns anyway? But you have to get you some, and even more importantly you have to GIVE you some. In the getting, you will see all the things you think you're doing well that you're not. You'll learn what you need to work on. In the giving, you'll learn more about how to make your own writing deliberate and conscious and the most common mistakes to be wary of in your own writing.
Read this blog: No, I'm not kidding. That's why I'm here. I write a blog about writing––maybe you've noticed. Given that this is literally what I do for a living, and I make enough to not die, I can't recommend me enough. Poke around. Put your feet up. Try the roasted vegetable polenta I just made for lunch. There's LOTS of advice here: writing prompts, craft advice, many many questions for the mailbox. You can't avoid hard work by reading a blog, but sometimes I can point out a pitfall or a shortcut and save you some time and frustration.
Okay, fine, or a blog LIKE this one. Or really any deliberate writing advice. The point is that you probably don't want to just write while sequestered away. You'll make the same mistakes over and over again, and while you will get better, your learning curve will leave a lot to be desired. You want to practice (as much as you can) but also try to make your progress deliberate. A self-taught writing expert isn't quite the anomaly that a self-taught concert pianist might be, but both probably could have saved themselves hundreds of hours of practice back at the beginning if they'd had someone show them a better way to do something basic.
For the would-be working writer or the ambitious hobbyist who dreams of one day "making it," there is no advice BETTER than "write every day," but there is a bit of advice OTHER than "write every day." I hope this helps. While it is likely to be a lot slower if not combined with the daily part, it may even get you where you want to go.