>because evolution would likely have spread them to everyone.
This is just your bias in what you think is more beneficial informing your belief that more intelligence = better survival. If this is the case, why aren't all apes just as intelligent as us? Has no ape simply ever had the luck to have a mutation that improved the carrier's intelligence? This is implausible. The answer is that one's environment determines what is selected for, and it is not the case that more intelligence is necessarily more fit for survival. Not even in modern times.
This is just your bias in what you think is more beneficial informing your belief that more intelligence = better survival. If this is the case, why aren't all apes just as intelligent as us? Has no ape simply ever had the luck to have a mutation that improved the carrier's intelligence? This is implausible. The answer is that one's environment determines what is selected for, and it is not the case that more intelligence is necessarily more fit for survival. Not even in modern times.