Swoleness has nothing to do with genetics. If you are in a caloric surplus for a long time with right protein intake and gym routine, you will gain size and become swole. I think the only difference is muscle insertions.
Majority of people in the Horn are nomadic people, these people travel days just find water sources yet alone have enough to eat. So imagine for generations these people were in a caloric deficit. Even those who lived stationary, the area for generations have been affected by drought and warfare. They are war ravaged people. Look at the Irish during their famine or Nigerian during their civil war, war destroys physique and healthy diet changes everything, Dutch were once the shortest in Europe but dietary change has made them the tallest people in Europe
The diet. The diet is heavily carb laden with not enough intake of protein and fat. Eggs, chicken, fish, steak and pork aren’t usually found in these areas, so people aren’t getting protein, even if they get enough protein they aren’t getting enough to grow.
Cultural isolation. The gym culture is non-existent in these areas and in urban areas, soccer is king, so weights are pointless. You have people in the new world have been gyming since the 40s and these people have been using steroids since. People in these areas would be shock to know that gym even exists yet alone steroids.
Number games. There is more Nigerians than people from the Horn. Now add to West African population/New world blacks and Bantu numbers, you would automatically see more swole other Africans than people from the Horn even if lifting was a big part of the culture. And ya cats are crazy, most African whether Bantu, West African and from the South are usually lean/thin built. This notion of HUGE swole Africans walking around is a fantasy
And honestly, these cats are straight lazy. They have no real incentive to work out and their culture promotes laziness. Khat, Shiisha and tea. They love to sit down and talk in the evening, instead hitting the weight, taking in the protein and working that body. Most guys originally start gym to "get chicks" but a lot of these guys don't care because they have number one access to their women and can always go back home for wifey, so no incentive there. But this is changing,
In my hood, some of the swole thick cats are actually some Habesha cats and even I know numerous few swole Somalis. But where I live currently, lifting is huge and is part of the culture.
I say wait in America, maybe just one more generation and you will see much more swole cats. Getting swole is very easy actually.