There has to be some environmental pressure which causes those people with the gene to be more successful at breeding than those without the gene. If a gene does not provide a dramatic increase in an organism's chance of breeding (or surviving to breed) then there's no reason for that gene to increase in the gene pool.
I don't see many examples of hyper-intelligence granting people increased breeding opportunities.
So in a society where intelligence was not valued - yes there would be less environmental pressure favoring greater intelligence.
But in general and in most human societies - intelligence is valued and intelligent people are favored by sexual selection - as well as general run of the mill selection - intelligence also contributing to affluence - which is also highly favored by sexual selection.
And there need not be a “dramatic increase” in reproductive success for a favored gene to reach penetrance in a population - just a minor incremental difference over many generations.