It's a lot easier to fix these things if it isn't voiced. 10 pages of non-voiced dialogue must be easier to do than 1 page of voiced dialogue, given the audio studio, scheduling, costs, and other arrangements voicing adds. Fully voicing everything is kind of a mixed sword. That said, it would seem like part of the solution would be to have generic dialogue voiced every game, then recylce as much of it as you can in the next game, adding to it. They don't seem to do that though.
Edit: That's not to say they couldn't have had the current system be better, but it would have to be more than just adding more voice.