Well, the thing is, there is no co-op as of yet. There's just been a general assumption that there will be in some time when someones gets around to modding it in. Because every reviewer (and every player) noticed that the game was absolutely screaming for co-op.
So...it's singleplayer for now. I just figured you'd enjoy it, seemed like your kind of game. And it's not like you don't have time to indulge in it.