How on Earth do you think players create characters without defining at least some of those in the process? Especially if they've all talking to each other about their character concepts and how they fit together? When I say "no backstory before play", I'm including the character creation (and worldbuilding, if relevant) as part of play. They've called "Session Zero" because they happen before the IC portion of the game starts.
....
Then what on
Earth are you talking about in terms of the problem here?
Nobody--and I mean
nobody--I've ever played with has had some "fanfic", as you so derisively put it, which they want to subject everyone else to....with the sole exception of a couple of GMs...who were very clearly wanting to build a world that all too closely resembled the fiction they'd written before deciding to GM.
When you rail so adamantly against any form of backstory that did not arise "in play", something you were
very specific to call out, how am I supposed to interpret that? I've spoken to
numerous folks, here and elsewhere, who say that and mean exactly what I described. Nothing, story whatsoever, until the dice hit the mat. It's quite possible; you literally just pick class and race (and, in 5e, background since you have to) and then go. Don't think a thought at all about who they are or what they're like, other than what you can glean from the ability scores you chose (or, more typically, rolled).
And I have never heard of "session zero" being seen as "part of play" in this way. It's called "zero" because it happens prior to play. Play doesn't actually begin until you, y'know,
play. Hence why people talk about the "character creation metagame"; character creation, and getting good at it, is one abstraction higher than actually playing the game itself.