I stopped playing WoW for my exams, and I had no motivation to start playing again.
The main reason I feel keeps people playing is Daily quests, these are quests you can only do once a day, with a relatively substantial reward. I don't know about everyone, but I know that many people, including myself, only logged on to do those quests as part of a routine. Add to that raiding, is incredibly social compered to most games, and once you complete a dungeon you have to wait a week for it to reset, effectively drawing out the content.
And with the issue of the pricing; I playing nothing but WoW (with other hobbies) for around 6 months before the exam season. That's around £45-60 for the gametime. I don't know many other games that give me 6 months of enjoyment for that price. Plus the expansions give an amazing amount of content for the price. So MMOs are cheap.
My friend, who never stopped playing during the exams (as I fear his results will show) just expected me to come back, since my "Guild needed me." Yes, I made friends on WoW that I happily talk to through other means.
Edit: And I'm not even touching on the reward/self improvment aspect.
TD:LR: Social aspects and drawn out, addictive, 'routine' content.