There is such a simpler explanation for this. You can get better at IQ tests by practicing and studying. For a long time IQ scores were seen as an important measure of your value as a human being, and more importantly, were used to gate access to jobs and other opportunities. So people learned how to get better at passing IQ tests.

Now IQ tests are no longer fashionable and we're drifting back to the scores you'd expect in a naive population.