They release a lot of additional free content (or used to, anyway. Not so much these days I believe), and the support burden is far higher than it would be for a release-once local game.
WoW provides Blizzard three sources of income (game price, monthly fee, ingame store) and I hear their support is quite bad. Many other MMOs get along just fine with only one or two while providing an equal or better level of support.
But Blizzard can get away with all that thanks to people like you buying their marketing crap.
On top of that, you can buy character boosts, too. No, this isn't covering development and support costs, it's shameless profit optimization.