As has been pointed out, it's like that because the origin characters are intended to be the protagonists of their own games.
In some ways, the custom PC is the Eder of the game. I am not really sure why Larian decided to follow along in the same ruts as DOS2, but I suspect it's because they want to differentiate themselves from the original makers of the BG series (Bioware/Black Isle).
Coop is important to Larian and origins are a brilliant design for that, as it allows coop buddies to easily take over companions in your.
