Because, ANYTHING you can do solo, is easier than being in a guild that does organised raiding (+ all the baggage that comes with that). Sure, MC is easy. Yet PUGs still blow up the raid on Geddon and fuck up the kill order on Majordomo. Shit guilds still spend 4-6h in BWL like we did in 2005, when my guild is clearing it in 55-65 minutes. Simply the fact you need to be a) in a decent guild and b) be active & good enough to have an 100% raid spot is MILES harder than coming home from school/work/Uni and hitting up some BG's for a couple of hours. One requires you to be a functional, social human being, the other can be done by bots.