If you have sub-Saharan African ancestry, that is N.EGRO ancestry, and you're born in the U.S. you're an African American. A majority of African Americans don't know specifically where in Sub-Saharan Africa their Black ancestors came from- this goes back to slavery times. That's why the term "African American" is used. A majority of African Americans are descendants of Sub-Saharan African slaves. When the slaves from Africa came over to the U.S. during the slave trade, Whites destroyed the records of where in sub-Saharan Africa they came from forced them to assimilate and give up their African IDs.
Most African Americans don't know where their Sub-Saharan African ancestors came from without doing a DNA test.
African Americans who are descendants of these slaves I am talking about also have some European DNA and/or Native American DNA - this goes back to slavery too. Whites raped the slaves and the slaves intermarried with Whites and Native Americans.
Black Americans have always been called "African Americans". This has been a general, collective term used for this group of Americans because most people in this group, as I said, don't know where in Sub-Saharan Africa their N.egro ancestors came from. There are some who do after doing a DNA test.
You DO have Black African immigrants- Kenyan Americans, Ghanian Americans, Liberian Americans, Nigerian Americans, etc.
With this logic, every person in America would have to place another country before America because no one has ancestors that originated from here. As far as people saying African Americans the term is only used because very few black people no what country they're from.