Cause they stay free… and get better. iPhone developers do tend to add more and more functionality to already free games. More for the same price. 
So for example if you checkout our review of popular Paper Toss game (see our review at http://blog.bar-coders.com/2009/08/06/apps-“ilike”-–-paper-toss/ ) you will notice screenshots for few months back. If you download the game now you will notice additional functionality on top of the one we reviewed originally. I wish the beer was like iPhone free games: Always free and constantly improving….. Maybe in an alternate dimension.