Why is it custom to tip for everything in America?

I recently went to the USA on holiday, and I couldn't believe how everyone tips for everything over there. Why? Why do Americans constantly overpay for everything? I went to a bar with a couple of friends and at the end I paid my bill for what it was charged. When the bartender saw my bill, he said "Thanks for nothing Ahole!" My American friends said that I should have tipped him. They told me that if somebody does a good job, regardless of profession, you should give them a little extra. That makes no sense! If they feel their work is worth a said amount, then that's what they should charge. Why on earth do I have to come up with some arbitrary amount to tip somebody in that country? Why can't they just charge for services rendered?