It's not, it's a cultural thing, even in Africa. There are probably socio-economic reasons on top of that (swimming is kind of a luxury for many people).

A colleague from Senegal (who didn't know how to swim himself) told me there were biological reasons as well (bone structure or something) but I'm highly dubious of that. I can believe that genetics can play a role in swimming proficiency but I doubt that it would prevent anybody from reaching very basic swimming skill levels.

It's like saying that black people run faster than whites, looking at the olympics it appears to be true but that doesn't mean that white people fall over when we attempt a brisk stroll.

I have never heard that here in Belgium. I also see black people in the pool quite frequently.