"Perhaps a few people had a TALENT and happened to be black. Now who would have thought that."
82 men have run 100m in under 10s. One of them is white. That is not random chance of who happens to have talent, that is statistically significant.
There are many things that could account for this, as many people have pointed out. White people are just as dominant in other sports such as rowing, equestrianism, cycling. These are much more niche sports that most of the population don't do (well, obviously people cycle, but not many do velodrome cycling), so whilst I have no evidence of the influence of genetics, it seems highly plausible that this is only to do with opportunity and exposure and not to do with genetics. In fact, in France, where cycling is much more high profile, top competitors are much more ethnically mixed - there top velodrome sprinter is black. We cannot know for sure until we have much more equal opportunity. I don't think it's true to say that no one questions why some sports are white-dominated - I think they do. I think the assumption (postulated by others) that white people 'should' be better and therefore we need to question why this is not so for running is an extremely minority view
But with sprinting this argument seems less convincing. Sprinting is high profile. Much more so in the Caribean than in other places, but it is still high profile in most places. Being "the fastest man in the world" has cross-cultural appeal, where being the best at dressage maybe doesn't. Many of the UK's sporting icons (Roger Bannister, Eric Liddell, Linford Cristie, etc) are sprinters. And obviously athletes of all ethnicities work exceptionally hard at what they do. But there are a lot of white sprinters devoting their all to it and not quite making the top tier.
Obviously, believing black people are good at sprinting is a ludicrous generalisation. Skin colour is far too broad a generalisation. But that does not mean that there are not specific genetic groups that are more likely to display particular physical attributes that make members of that group more likely to be physically adapted to a particular activity. If this group is black, then this can give the impression that black people are good at this when in fact it is much more specific. But looking underneath that to explore the role of genetics seems to me to be valid.
I understand that this is a very sensitive subject, and that much pseudo-science has gone on over the years to demonstrate (pretend) that oppressed minorities are 'not as good' in some way. Maybe this is so extreme and so sensitive that it is really impossible to talk about this issue at all without being perceived as racist and/or distressing people. But I do not see why it is not a valid question.