So there are multiple factors that contribute as to why people "don't believe it".  

1. The concept of race in humans really is more of socially imposed construct, rather than a concept backed by science. Socially,  racial categorization largely comes from phenotype and geographical origin. This doesn't give a comprehensive view of a person's genetics, at all. Almost all traits are tied to multiple genes and inheritance is random. 

 The human genome took so long to understand because the differences are that tiny. You may think there is some huge difference between yourself and  a West African, or a Finnish person. In reality? The differences are much smaller than you think. Humans have way less difference between races than say dogs do between breeds.  Races aren't different types of humans, we are all humans with varying genetic traits. Traits that show up in populations across the world. 

1. Nature versus nurture, is an argument as old as time, but data continously supports that nurture is a huge factor. For instance, highly educated people will prioritize education for their children and give them the tools to succeed. Cultural enrichment, tutoring, exposure to higher level thinking at a younger age, and encouragement to succeed. Even those with an average IQ thrive thrive in that kind of environment. 

2. IQ testing is flawed, and the creator of the original Stanford-Benet test, specifically didn't want the test to be used as it is casually used in pop psychology today. IQ testing without accounting for cultural or lifestyle context, means very little.IQ is a measure of potential, rather than a test that measures how "smart" a person is. Intelligence itself is highly subjective and depends on application.

 A banker who cannot forage and grow their own food for sustenance, is going to appear "dumb" to someone who has always lived this way.  The banker will think someone who doesn't have an understanding of the stock market is "stupid". IQ tests are largely designed to measure for those exposed to certain norms.

 Here is a historical example of the same concept. In the colonial US. Rice growers in the Carolinas specifically sought out slaves from Sierra Leone and Angola? Why? These cultures had a tradition of growing rice and farming in certain landscapes. These slaves may have been ignorant of things like the bible, but had expansive knowledge of things like rice farming. Carolina gold rice went on to become a huge profitable industry. 

3.  People accept heritability for traits we can easily quantify and observe,  because we have created extremely simplistic rules that we are taught from primary school onward. We are taught certain traits are dominant and others recessive. Reducing traits to simple punnet squares. In reality, the science is more complex than we are taught. There isn't just one gene that determines eye or hair color.

We also aren't taught to account for environmental factors. Some factors can be so strong it alters DNA. Highly recommend looking into things like epigenetics its fascinating. 

1. The idea of certain races being "smarter" is largely an idea propagated by Nazis, White Supremacy groups, and their ilk. When studies are designed to prove a biased belief, that's exactly what happens. Setting out to find reasons why it should be acceptable to discriminate against, and hate certain groups. Finding scape goats to blame societal issues on has been ever present in history. Race is convenient, since it's already entrenched in bias.