When I was young and solely a player, I wrote long backstories that were basically just for me- though I didn't know that at the time.
As I got older and moved further over to GMing, I cared less and less about what happened to a character before gameplay.
I'll have a sentence or two and figure out the character from there- and more often than not when players hand me page-long backstories I am unlikely to read them... I tell them this now, though, after receiving too many "I am a self-exiled warlord-prince that stands to inherent an army when I choose to return to my people."
And usually they have any conflict already wrapped up in them so the character isn't saddled with any possible exploits that I could've used if I wanted to

"my elf bargained her soul to a demon for power to stop her parents from being killed, and then she did save her parents and then I gave the demon the would-be murderers soul and he was happy with that so now I'm an adventurer."
... It would have been much more usable if the fiend was still promised your soul.