It's so that people don't just burn through all the content, u may not like it but from a business PoV it's actually quite intelligent. It means that people who thought they had burnt through it all come back and play each time they find out there is something new to do. It's all about stats; 'like did you know that we had over a million people log in this week? Doesn't that just show u how awesome this game is?' It's stuff like this that earns game companies bigger budgets for there next game.