Because the AppStore does not consistently deliver good apps. Some are great and a steal at that price, some crash your phone, some are so bad that you feel cheated even though you only spent $2 and 5 minutes on it.
If there was a decent chance your $2 bought a rotten tomato thrown in your face, you'd be hesitant to pay as well.
If there was a decent chance your $2 bought a rotten tomato thrown in your face, you'd be hesitant to pay as well.