The issue with putting in established lore is that you have to actually show that lore.
In a game like Mass Effect, this is done by having cutscenes where other people are talking about your character at the start of the game. In Dragon Age: Origins, you have the prologue where you're doing your origin story.
Neither course would work with the way BG3 is set up where the very start of the game is you being tadpoled (Which is suggested to impact character's abilities - Hence people like Karlach, Gale, Lae'zel and Wyll starting as level 1 and barely able to 1v1 a Bugbear despite being highly adept warriors/wizards - Thus it would be super awkward to have a prologue where you're "Full power" before being abducted and tadpoled)
There's also the secondary effect of things like having choices a la ME's backstory options, as you'd have to create different dialogues based on what was picked, which is additional work (ME gets away with it due to how little it's actually brought up, just a handful of references across the entire trilogy and 1 unique mission in the first game).
Meanwhile, blank slate PC can work as people can create their own headcanon for their character and try and pick from generic responses to try and push that headcanon.
It could be nice to have options for backgrounds, maybe nothing as concrete as an Origin character, but things like what city you're from, your profession (Aka the "Background" option being somewhat relevant other than the skill proficencies) and a Solasta style demeanour choice to influence your speech... But such things would be a lot of work and you'd still end up upsetting people.
