Easy question, easy answer. Games cost a magic set 15 dollars a month or 5 dollars for Free Realms for the kiddies because... wait for it... that's what the market will bear. If you wouldn't stand for it in big enough numbers, they wouldn't do it. Simple as that.
What's the best pricing model eva'!?! Pay as you play. Why don't they like this model? Cuz the chances are that they will win out with a monthly sub that you forget to use enough. Simple as that.