Socialization = Difficulty
MMORPGs are easy games. Typically, all you need to do to get ahead is spend time playing. Most games lead you to max level along a fairly linear path, punishing you with the death of your character if you stray off course. And really, all a death means in a MMORPG is that you need to spend more time getting back to where you were before the death. Most MMOs are carefully crafted so that the solo player always experiences content suited to their relative power level. As long as you can click buttons in a certain order, you win.
MMORPGs have to be easy thanks to their business model. Successful subscribers are happy subscribers. No one would pay $15 per month to have their virtual ass continually handed to them by a Large Spider. Well, some players might, but that's a topic for another blog entry. If players are always gaining wealth, power, levels, or items, then they are happy. If a game is difficult, then they get rewarded less often and thus aren't happy, so they unsubscribe.
But there has to be something in the game for players who want a challenge. There has to be awesome rewards that set you apart from other players. This is where the game forces you into socialization. Want to finish that Elite quest? Get a group. Want to go raiding? Form a guild. Want to stand a chance in large scale PvP? You'd better have some friends. The biggest challenges and biggest rewards a game offers requires that you socialize with other players, and that socialization is often more difficult than the baddest boss the game can throw at you.
Face it, some people just don't work well in groups. They don't pay attention, they don't follow rules or tactics. They don't do the work required to fill their role in a raid. They don't listen to TeamSpeak and they're constantly AFK. We've all suffered these kinds of fools. How many of you have thought, "This would be SO much easier if everyone would get here on time and pay attention."
PvP teams in Guild Wars are more successful if PEOPLE communicate.
Bosses in WoW go down faster if PEOPLE pay attention and fill their roles well.
Corporations in EVE simply cannot prosper if PEOPLE don't contribute.
Perma-parties in FFXI level much faster because the PEOPLE in the party have a vested interest in getting ahead.
So in the end, when a game developer wants to add challenge to their game, they create situations that require players to work together, to socialize. Naturally they can create bosses with more hit points and other such things to make them more challenging, but bosses have to be able to be killed and PEOPLE are often more challenging than the boss itself.
Do I think that MMO developers should ban group content because its sometime difficult to find good people with whom to group? No, of course not. I just find it interesting that other players can be just as big an obstacle to success as the meanest dragon or toughest grind.