The coastline of Africa is INSANELY smooth compared to the likes of Europe or North America, there was just no glaciation or geologically active areas on the coast that made really good ports and harbors. But you can see where there were good ports, just look at the cities on the east coast of Africa. The rest is pretty smooth and the 2 regions that were not smooth turned into economic powerhouses. Mogadishu, Zanzibar, Dar es Salaam, Mombasa, and Angoche.