NPCs having their own lives would take the "you are the end-all be-all of the world" feeling away from the player, and sadly, there are plenty of people out there who want to feel coddled and special and wonderful when they're escaping into game worlds.
Because no matter how much we rail against it, there ARE a fair few people for whom gaming is an escape from their own insignificance---having even the video girls tell them they're already romantically involved might just push them over the edge.