Why do some "White" Americans call other Americans: African American, Mexican American, Japanese American,etc?

I agree, I am a Native American and I can say that anyone living here is an American since they reside in America. This invisible boundary is nothing but a separation of consciousness to create cles and types of people. Wars are started because of ignorance like this since it becomes "us versus them" in the general mentality but the simple fact remains that we are all children of the Earth and no matter what title others may use to separate us we are human collectively.