What is ironic is that your fundamental premise is in itself partly racist by lumping ALL PEOPLE OF AFRICAN HERITAGE into the same boat.
Most ethnic groups of Africans share the same ectomorphic (long arms, long legs, short torso) body morphology that is conducive to speed. West Africans, like the Igbo of Nigeria, via an unintended consequence of a genetic trait that originally aided in malaria resistance (sickle cell trait) have picked up a more efficient anaerobic metabolism. East Africans, especially groups like the Kalenjin and Oromo via recent migrations to altitude and very slender body morphology, are more suited to distance running. It is important to note that the AVERAGE West African and AVERAGE East African will only be a bit better suited for sprinting or distance running than the average non-African counterpart. However, the outliers stick out much more profoundly because such small difference in genetic talent determine how things shake out at the top. It should also be noted that this long-legged, short-torso body shape is a huge disadvantage in swimming, which helps explain why swimmers of African heritage are relatively scarce.
Africa is an amazing specter of human genetic diversity. There is more human genetic diversity on the continent of Africa than in the rest of the world COMBINED. Some ethnic groups, like the Pygmy of central Africa (short height, short limbs) have traits that have them perfectly suited for their environments that will inhibit success in professional athletics. You just can't make blanket statements about Africans and say they will all be good at sports. The genetic profile is way too diverse. However, certain groups have genetic profiles that really help in certain sports. The Kalenjin of Kenya are truly an amazing example.
Kids self-select with remarkable efficiency. There may be cultural pressures but really, kids like being good. And the truth is that genetics matter, and the average genetic profiles of different backgrounds means some groups will be better at some sports than others.
You really should read the Sports Gene by David Epstein. The explanatory power of the book is simply amazing, and shattered any remnants of belief I had that athletic talent for different sports was inherently equal across ethnic boundaries.