I can't say why you play MMO's. But the reason I've played WoW for about the same time as you is that it's an easy, quick way to socialise with an easy escape if it turns to crap; you just move to another server. The gameplay itself doesn't keep me playing, it may be a factor for a while but eventually it's the people I play with that keep me paying the subscription.
It's a whole lot easier to sit down, log in, play for a while and meet a few people than it is to go outside, find an activity you enjoy and can do with others and meet even 1 person you get along with. Even then, if things turn ugly for whatever reason, it's a lot more difficult to start over somewhere else in real life than it is in an MMO.
That's pretty much why I play MMO's; it's an easy way to socialise with very little risk.