My favorite part of a blank slate custom character is that their background is whatever I imagine it to be. I don’t need anything else. I don’t feel like my characters are less integrated into the story. My character is important because they are the one who has assumed the leadership role and is making all of the important decisions.
Having DAO origin stories would limit our choices in character creation. BG3 has so many more race and class options than DAO that origins for all of those would be unfeasible. DAO originally had twice as many origin stories planned out but BioWare had to cut them for all of the resources they took to create. There is a reason they dropped the idea from DA2 and DAI.
It’s also worth noting that we don’t yet know how integrated custom characters will be into the city of Baldur’s Gate. It could be that custom characters will have their own homes and some NPCs that reflect their class.