One of the core reasons behind this is the question of reliance. The community of group-based games tends to be stonger than that of games where you can be almost wholly independent, since you rely upon others to work with you. By way of contrast, examine some of the behavior found in World of Warcraft's random dungeon tool, where you find yourself working with people whom you're statistically unlikely to ever see again.
One of the laments about solo-friendly games is the death of community, and while that's not altogether true, Heimburg's post certainly makes a number of compelling points. Well worth examining if you're interested in building a community or just in the ways groups develop.