Why do Americans feel that they have to tip every time they buy something?

I live in England and from what I know, over here we really only tip when in a restaurant and that's only if we feel like it. But from what I understand from watching and reading about America is that you see tipping a person for whatever reason as obligatory. Why is this. At the end of the day that person still earns their wage so why should you give them extra for something they're being paid to do anyway.