People identify themselves on how they perceive themselves. It's not exclusive to African Americans the same could be said for people that identify themselves as Irish Americans, Italian Americans,Scottish American, etc. As I've said it's all in how people see themselves. I had an older African American teacher that would refer to herself and others as Negroes. Did it make me uncomfortable hearing that? yep, but that's what she grew up with. I grew up with African American and black as descriptors. Do I like it, not particularly, but when's the last time you saw just "American" on a survey or form?
You are correct in that all these subcategories separate us, but what exactly are we doing about it? We have an occasional hot issue about it, then it dies down and we go back to our separate little lives until the next hot issue. It's getting better but there's still some work to do. People can relate to each other (we all need the same things in life, experience the same joys, pains, and sorrow), but it's hard to come out of our comfort zones to do so.