The thing is that the origin characters have a preestablished backstory and personality. You can get a whole cinematic with them where they explain their goals, story and desires in the character creator. The custom character option is there for people who want a blank slate they can make their own. Wiping away the origin characters' personality completely just defeats the point of playing them. There are still roleplay elements and choices you can make for them, but you're playing as a character with a prewritten backstory and personality. Similar to Geralt of Rivia or Arthur Morgan.
Personally I haven't really enjoyed playing as origin characters because I feel it takes too much of them away, like I might as well just be playing as Tav.
But there is an option to completely turn off your character reacting to things as you explore. It's under "gameplay" I think.
