Actually if African Americans took DNA tests they would probably find out that their closest relations are other African Americans and then people in the Caribbean; particularly the Dominicans, Haitians, Jamaicans Puerto Ricans and Cubans. The reason for that is most American slaves were taken to the Islands to be broken and made into slaves before being brought to the USA; because no one willing becomes a slave.
My family has lived in the USA for hundreds of years in Virginia, North Carolina and Tennessee. When I took a DNA test I learned that I have Dominican ancestors, but no one in my family has any recollection or family records showing that we lived anywhere except Virginia, North Carolina or Tennessee. And we have some family historians that keep pretty good records.
I don't know why some slaves were brought to the USA and why others were left in the Caribbean; but DNA is pointing out that the people in the Caribbean are our closest living relatives; even closer in time than the Africans.