Because there are much more important effects on intelligence or skills than where one's ancestors were born.
There is no significant correlation. There was one book published a few decades ago that made the claim, but it was neither statistically significant nor larger than a rounding error. The difference in IQ tests -- which are known to have irrelevant biases -- was a few points. Variation within populations was several dozen, and measurement error was several points.
That means the conclusion was intended to sell books to people like you who don't understand it. It has no validity whatsoever. It is a null result, fully consistent with no difference at all. And just as predictive as a sun sign or other BS in predicting one's skills. That is, not at all. Racist claptrap, nothing more.