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, and 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.
What makes your character so special that they assume leadership with such ease? What if you want to play a character who isn't the leader?
How can you consider your character as integrated into the story when you'll be dealing with your companions personal questlines while your own character's history is never more than a throw away line?
Of course I'm assuming the Custom MC won't have their own personal quest generated by your choices, but I think I can consider that a safe assumption.