Why not? Why do some cultures stress working hard in school and some don't? You're trying to rule out inherited intelligence out of hand. It's probably the biggest reason.
I have an IQ over 120 and have a college degree. My sister has an IQ of 95 and cannot read above a 4th grade level. Her son, my nephew, has an IQ of 151 but hates school and wants to be a mechanic. Because my nephew’s parents were drug addicts who abandoned their son at age four, I reared him. So, where did that high IQ number come from?
I do not consider the “inheritance” of IQ as being the only factor in the intelligence any group BECAUSE it is one of many inherited and environmental factors in a group of people. As I’ve already said, the intelligence bell curve for an ethnic or racial group may indicate a cultural attitude towards education, the result that “some cultures stress working hard in school” while others do not.
In the United States we have groups of people who came here for a variety of reasons. People who emigrated from China and Japan did so because they sought opportunities not available at home and wanted to improve their economic success. Black slaves were forced here against their will, and those with leadership ability, generally associated with intelligence, were murdered to produce a docile population. The dynamics of a group’s genetics is the result of the survivors who reproduce within that group.
What I said in the above paragraph may have a great deal to do with the difference in IQ between Asians and Blacks. That doesn’t mean individuals with high and low intelligence only appear in one ethnic or racial group and not the other.