So if you do well on tests, then you probably would do well on the IQ test as well as tests in school, which means you are more likely to have attained a higher level of educations, which is associated with a higher income.
To actually prove to me that IQ is a thing, you would have to show me that what you call IQ is innate. Meaning that you can't do better on an IQ test if you study, which I don't believe is true. All written tests I have ever taken have tricks or patterns that you can exploit after some practice. And then you'd have to show me that IQ is a better predictor than some other rote test to tease out general test taking ability or ability to sit still.
A lot of people just work off the basis that IQ is a thing that we can measure, which I don't believe to be true, hence the pseudoscientific swindle