So it's reversion to mean which is often seen in nature. If we see that today we don't really need high IQ to navigate our way through the cities, nation or even continent because we've GPS and other technologies which drastically reduce the amount of complexity we need to keep up with. Why should a higher IQ exist then?

If we look at the past and see when and where the IQ increase took place, we see it increasing when when we were required to complex terrain and protect ourselves from harsh climate.