What's wrong with playing them like singleplayer games? The appeal is the persistent, shared world not necessarily playing with other people.
From the developers' point of view, it seems like a lot of effort (to create the shared persistent world) for not much return, if everyone's playing in that shared persistent world just to have a sense of "background hum" of other people being in the game, and not actually interacting with each other.
I mean it does work to make the world feel more alive, to have other people running around in it. They behave differently to NPCs, and peripheral awareness of that does make the world feel more real. But is it worth creating a whole game just to capture that feeling?