First, because minds are tremendously complex and much less well understood than things like fast and slow twitch muscles. That we see different distributions in one does not imply different distributions in another.
Second, because we have tremendous evidence of clear bias, discrimination, and oppression of certain groups and a direct link to confounds that make claims about people of certain races just being genetically smarter amazingly messy.
Third, because for physical characteristics we don’t use broad racial categories but instead use ethnicities. “Black people” don’t have the specific physical characteristics that enable the absolute peak of running capabilities. Specific subgroups do. Expanding to racial categories (that were invented by humans to justify slavery and colonialism) is fraught with peril.
Fourth, because discussion of physical traits usually looks at the very very very peak while discussion of intelligence is usually a broad claim about all members of a race. This is not the same thing.
And finally, because these arguments have been used for centuries to justify literal enslavement. To justify denying voting rights. To justify all manner of horrors because it was just obvious to eugenicists that black people were less developed. And we should be extra careful of claims that, if followed carelessly, lead to genocide.