I would guess this decision is due to Larian putting a lot of time and effort (+$) into making the origin characters and they want people to experience them.

That said, I would love the option to use fully custom parties, particularly for later playthroughs of the game.

Hopefully they add this option along with a photo mode in a future patch/update.

He who fights with monsters should look to it that he himself does not become a monster.

If you gaze long into an abyss, the abyss also gazes into you.