If the question is about why some of them are not, then the answer would be the same as for everyone else: coincidence of other factors. And even with them in mind high IQ means higher averages, e.g. bottom 95% income at the higher IQ on average is higher than bottom 95% income at the average IQ.
https://thesocietypages.org/socimages/files/blogger2wp/2Meth...
If the question is about why some of them are not, then the answer would be the same as for everyone else: coincidence of other factors. And even with them in mind high IQ means higher averages, e.g. bottom 95% income at the higher IQ on average is higher than bottom 95% income at the average IQ.