It's not common knowledge, but adding voice actors to games is VERY expensive, plus not everyone likes characters to talk.
Furthermore, no matter how advanced AI is, it still has that artificial tone that you immediately detect when you hear it, and it's limited to certain voice types. You can do it yourself with AI voice modifiers, but again, that falls back on the first problem: it's expensive.
There is a concept in production that talks about “doing more than what is asked for,” and it is also the reason why many AAA games are so ridiculously expensive to make, in terms of their retail price and long development times. I mention this because adding voices to AVNs falls into that category. It's not necessary, most people don't even ask for it, and many don't even like it. You delay development by doing something that doesn't even improve the value of the game.