One aspect is that there's at least partial causality the opposite way. While earnings are correlated with IQ, they are even more correlated with your family earnings/socioeconomic status (SES); and there are many aspects of how low SES screws up childhood development and education (on average, not necessarily for all individuals) that would result, among other things, in lower IQ test results. I.e. it's a reasonable hypothesis that the correlation implies that low intelligence makes it hard to be rich just as much as being poor prevents one from developing and applying their intelligence.