I kinda see your point. The warm feeling of knowing a real human told me "die, bitch!" isn't a feeling I've ever taken away from playing UT.

On the other hand, lots of AI-generated VO is very easy to spot, and sounds awful. It stands to reason it could meaningfully take away from even a completely plot-free game. If I were a voice actor, I'd feel insulted that anyone would find it comparable to my work.