It's the same reason why you don't see any intricate dialog trees in games like Dragon Age or Fallout 3 anymore, where they feel this need to have every single line spoken out, and have to make their recording-sessions as cost-effective as possible, so they make sure that very few lines lie in an obscure branch of dialog that may never be heard by the player.
This wasn't a problem in Baldur's Gate, Planescape:Torment, or even Neverwinter Nights and the like, where only the greetings or long monologues were voiced-over, with the rest of dialog being in pure text, and lots of it.
