I wonder, do you understand what being raised in poverty does to a human brain? What poor nutrition and noise do to a child's learning? And that these impacts are not readily reversible without concentrated effort that is far beyond what most people can afford?
Quite simply, as per Scarcity (by Mullainthan and Shafir: well worth a read), a low-SES upbringing will suppress your IQ by approximately 15 points as compared to what you might expect based on genes alone. Consider that the average IQ is 100, and that 15 less than that is 85, and 15 less than that is the starting point for definitions of mental retardation, intellectual disability, borderline intellectual functioning; whatever you want to call it.
Yes, non-US Africans do well, but they are a self-selected minority of (I would wager) middle- and upper-class people from their countries. As such, they have not been brought up poor, with the attendant deficits in nutrition, education and (at least occasional) peace and quiet. For example, I used to work alongside a Nigerian woman with a PhD in computer science. Her father was very high up in the pan-African oil industry.
Do some people with low-SES backgrounds thrive? Of course, there are always exceptions, but African Americans have 1/10th the accumulated generational wealth and are three times more likely to be in poverty as their white counterparts.