How come RPG games are not multiplayer?

Now admittedly, the first rpg game I ever bought and played was Mass Effect 2 for the Ps3 as a matter of fact I am playing it now and loving it, but it has come to my attention that multiplayer for rpgs is unehard of? But why? I think this game would be great for split-screen co-op multiplayer at least, given that most rpgs play like this, how come this genre is always single player?How come RPG games are not multiplayer?
MMO's are basicly rpg's, i do agree mass effect would be great for multyplayer, but the point of a rpg is to sell the story, and make you belive you are in it. I dont think that would be possible if there are two sheperds running around (although if you could controll a squad mate that would be cool)



So in short if you are looking for a multyplayer rpg most MMO will work (such as lord of the rings... and its free and fun)How come RPG games are not multiplayer?
RPGs do have a lot of multiplayer games in the genre. Story driven games though are usually better single player. All single player games that I have played have had better stories than the multiplayer games.