I don't know what studies you are looking at, but African Americans are not overwhelming descended from Yoruba and Igbo ancestors. Some Igbo were taken to Virginia and some Yoruba were taken to the USA and the Caribbean; but by and large African Americans are descended from the Mande people from SeneGambia, and the Windward, Gold and Grain coasts (originally from the Kingdom of Mali) and the BaKongo people of the Kingdom of Kongo and Angola. If you can find an African American without a strong mixture of the people from those two regions then you have really found something.
African Americans are overwhelmingly from the regions of Senegambia, Windward Coast, Grain Coast, Gold Coast and West Central Africa, which encompass the modern countries of Sierra Leone, Liberia, Northern Ivory Coast, Northern Ghana, Mali, Burkina Faso, Gambia, Senegal, Congo and Angola.
Btw, there is a historical reason for that as well. It is because the slave owners in South Carolina, North Carolina and Georgia wanted people from Upper Guinea that were knowledgeable of rice production. The people of Kongo were brought in because the slave owners feared slave revolts and they tried to get slaves from different ethnic to lessen the chances of having revolts.
African-American mitochondrial DNAs often match mtDNAs found in multiple African ethnic groups
Senegambia, the Gold Coast, and the Bight of Benin - U.S. Slave Trade - The Abolition of The Slave Trade
NPS Ethnography: African American Heritage & Ethnography