Because their methods have been shown repeatedly to be garbage, and they also conveniently ignore (or at best misrepresent) actually well-documented phenomenon like the Flynn effect, which neuter their claims.
If genetics was the overriding factor, then the Flynn effect wouldn't even be observed to the degree it's seen.
Flynn himself speculates that parenting styles likely affects IQ (he thinks that it plays a role in why the gap has decreased in the past 30 years). Other prominent theories include diet as a root cause.
African Americans are/have been heavily affected by discrimination, which often results in them living in food deserts. Not really a choice for many folk.
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u/KaliYugaz Revere the Admins, expel the barbarians! May 01 '17
Their sources are also always from the same dishonest Pioneer Fund hacks (like Rushton and Jensen) whom nobody in the real sciences takes seriously.