Different people game for different reasons. If you're going to play with them, you need to accomodate that reality. Not everyone plays for deep immersion or identification with their character. Some just like to blow off steam tossing some dice and killing stuff, or playing around with their chosen character build, or many other alternatives.

Some people like to develop their characters in chargen, others prefer to do so in play. Usually that's a continuum, rather than two polar opposite positions. This is a long-standing bone of contention over which is "better", I just say they're different preferences.

For some people the character comes alive in play, and no amount of backstory aids that. For others writing backstory helps them get a grasp on who this person might be.

Me? I've come full circle, I don't want extensive backstories of stuff no one is likely to read or care about. Indeed I prefer a solid concept over some fiction about stuff that didn't take place in play.