You're completely right. I have no reasons and yours make complete sense! My only reasoning for it is my lack of creativity and inability to role play other characters. As an example I thinking of when BG3 finally allows us to play origin characters. Having already played as custom character in the EA we have come to learn about these characters as companions. If I were to try and play say Shadowheart as my player character, it would be hard for me to reconcile how I want to play her with how I know her as a companion. I would have much preferred if Shadowheart was just a companion or just a origin, so I wouldn't have any predefined notions of how she should be roleplayed. For another example if I'm playing Astarion and I have a choice of being selfish or selfless knowing that as a companion he tends towards selfishness, it would be weird if I took the selfless choice even if that's the one I want to make. For most people this is probably a non issue.
