Main voices and significant side characters of the story, I agree. But for some random NPCs that just add to the ambience of the environment, I think A.I can fill those roles. It just would be too cost prohibitive to hire actors for every single character voice. The developers would just not do it or they just repeat the same dialogue over and over again and that gets monotonous.
I say this as a writer who also used to work in videogames.