what, in your opinion, separates "fic" from "stories"?
lets set aside the obvious square::rectangle analogy and get to the real question you're asking, which is 'what is your problem with fic'.
which is technically not even the right question, because i don't have a problem with fanfiction as an activity. my only problem with it is as an end.
fanfiction is not serious writing. you can take it seriously, it can be tonally serious, but at the end of the day, by virtue of it standing on another person's shoulders, it is not serious writing. there is still that little bit of development you're opting not to do, there is still that little shortcut you're taking. there is an expectation of your audience that they will engage with it because they liked the thing you're riffing on, and not because they like you. they might engage with it because they like your ideas, or have read your other work, or your prose is legitimately good, but these are happy accidents. your work, no matter how good it is, is not standing on its own. and that's why it's not serious, the way playing covers on guitar for your friends is not serious. it's not a bad or lesser thing to do. but you're only going to get so far with it.
now, are there exceptions? of course. wicked (the maguire novel) was arguably a fanfiction. is all writing that isn't fanfic therefore inherently serious? of course not. i've read some garbage not worth the paper it was printed on. i don't want to be misconstrued as saying i think coolsville sucks, that fanfic is a waste of time. fanfic is perfectly good as a hobby, as a way to connect with others, to engage with or critique existing ideas. it just can't be where you stop if your desire is to grow.
so when i say fanfic is not serious writing, and when i say i have a problem with fanfic as an end, and when i say my writing advice is not directed at fanfic, what i am specifically saying is i want you 🫵 to want more for yourself. if you're passionate about improving your writing, if you're going to work hard, it should be for yourself. your ideas. your own work. your own message you want to convey. your own characters you care about. your own perspective. you can only get so far when you squeeze all of those things through somebody else's lens, and i want you to go as far as you can!
but again. i'm putting my hands on your shoulder. again. i am not saying fanfic is a waste of time or an embarrassing pursuit or whatever slight you want to take my opinion as. there is inherent value in every creative pursuit, and if you only want to write fanfic and nothing else, i am powerless to stop you. but my advice is not for fanfic. my advice is for you.