I never require a backstory for multiple reasons.

1. I like the atmosphere of RPG's when they are friendly, laid-back hang-outs where everyone's cooperating for everyone else to have fun. I really dislike the drama that arises when people at an RPG table starts judging each other based on their roleplay style, like "ugh, Alice never has a consistent personality in her characters" or "ugh, Bob always plays the same character" (I do make an exception when someone's using roleplaying as an excuse for toxic OOC behavior). So I don't really want to force players to come up with backstories, and I certainly don't want to put any requirements on their backstories for me to look through and judge.

2. A lot of players have different roleplaying styles, and I don't want to constrain people who prefer to improvise by demanding they come up with extensive backstories.

3. I think backstories do kind of burden some roleplayers. In establishing a backstory for their characters, some folks tend to flanderize themselves in-game by keeping to that backstory to an unreasonable degree, so that they'd actually be better roleplayers if they didn't tie themselves down with it.