Whatever else is true of raiding, it is definitely social. Communities spring up and keep going largely based on that raiding community, to the point that it's easy to assume that everyone in a game's population raids. There are lengthy discussions about raiding, about how to raid, about tips and tricks for clearing raids. The social aspect of raids is what I think has kept them around so long; it's easy for a designer to look at that sort of engagement and see it as vital.
Yet there's more to the story than might be available at a glance, and the social aspect is not without steep costs. Those social elements do not carry the weight of everything else... mostly because they aren't as strong as they appear.