Making other people (ie my friends) conform to my schedule, or them making me conform to theirs, has been the nr 1 reason why so many of our pen 'n paper role playing sessions never came to be. I want to play video games on MY terms, when I want and when I have the time for it.
But if videogames are to be a social activity for me, then it'll take place in an MMO where I can pick and choose who I hang around with, and take plenty of time to get to know them better before I commit to any group activities.
In a non-MMO game, this simply doesn't happen. Picking up random strangers for a co-op game is setting yourself up for massive dissapointment and frustration. It's just like getting Pick up Groups, and I believe every MMO player who's even remotely into dungeon running knows what a disaster it often means. Sure, you'll cherish the successful moments more, but in singleplayer you simply don't have to deal with that shit. Less pain and more fun times.
Short version: You NEED people you can rely on in a co-op game to have fun. Strangers just don't cut it, because their demographic is, by its very nature, unreliable.
And if we can't have fun when playing games, what's the point?