the same thign happened in OS2, the reason for this is that evey NPC can also potentialy be the main character.
when you pick the NPC for the dialogue you essentialy turn the NPC into "Main character" mode.
Then apply the same logic that it alked about before.

Im not even gonna pretend that saving costs for mocap and voice acting isnt also a thing here, but there is a justifiable reason

It personally takes me out of my character concept when i only got one or two voices to pick from. It gives the character a predetermined personality and i dont lik ethat.
Especialy when i play classes like Barbarian that quite frnakly dont work without a heavy austrian accent