W e l l . Most Asians/Latinos do that because their immidiate family are immigrants. For me, for example; I'm brown - my mom and dad are immigrants from Nepal but I was born in the US,,, so when people ask for my ethnicity I say, "I'm Nepali-American", which is true. I can say this because my immidiate family came from foreign countries. Now this is how most Asians and Latino people are, they can at least track down that their family started living in the America's when their great-great grandmother migrated here for god knows what.
But it's more difficult for black people to do the same because of .-. w e l l .-. because their ancestors were brought to America as slaves. This basically destroyed the chances of modern day black people finding their ancestral roots because slaves were not, yeknow, they weren't considered human enough to keep track of culture and History. This f**ked black people even t o d a y up, because listen, you could ask a white person where their ancestors came from and they could trail it down to Irish or Scottish or Russian, or some European country. cuz white. Basically it's harder for black people to go like, "Yes I come from Nigerian roots" or, "My ancestors came from Jamaica" because their blood History was erased by crackers.
SO, because black people have settled down in America for so f**king long, people just narrow down their ethnicity to "African-American". Everyone is too ashamed to admit that they can't find out their specific roots, because Europeans snatched it all away from them///