Why are African-Americans referred to as African-Americans and not just Americans?

it's stupid...most African American were not even born in africa. I call them blac...i call myself white....i call asians asian. I am not racist, but honestly, the whole thing is so dumb. As long as what you are saying is not offensive, it should not be an issue. If a black person called me white trash or something though, i feel i then have the right to call them a ******. I still would not, because it is very childish. I saw a little black kid in my high school (when i was in high school) wearing a shirt that said "i don't talk to white trash".......It was funny because none of the teachers cared...but if i would have been walking down the hall with a shirt that said "i don't talk to ******"...i would have been expelled. It is so childish., people are all equall...we should be treated that way.