They aren't indistinguishable. As you noticed, they basically have their own accent because their speech is so stiff, they can't intonate properly on the right words to convey the right emphasis and connotation, and if you listen closely you can even hear the combination of tones over-lapping each other incorrectly.

That being the case, I would expect that you would want script lines to be used for many bits of filler or useless dialogue, but you still want voice actors to act, as good ones have a mastery of linguistics, emotion, and noise.

I've seen games that were heavily AI, and actually voice acting is the worst feature of them.