The borders of African countries were drawn arbitrarily by the colonialists, throwing disparate people's/nations with nothing in common together. The Turkana are as different to the Embu as the Japanese are to Spanish. The Luo are as different to the Somali as the Inca are to the Russians.
Worse, the Beberus adopted an active divide-and-rule approach. So in Kenya for example the Kambas were designated 'loyal' and tonnes of them were recruited into the colonial police and army, and the Gikuyu's were labelled unreliable thieves. Guess who were used to fight the Mau Mau (clue, who led the army until 1984?). Today, those two very-closely related communities still view each other with suspicion.
The mistake we keep making is assuming that because we as Africans have a dark skin eti we are the same. Don't Eskimos, the Chinese, Japanese, Germans, Russians etc etc also have the same 'white' skin? Are they the same?