It’s called a hardy weinberg equilibrium. Unless there’s some catastrophic event, like we kill all the redheads, or something less sadistic like some kind of geographic change like a flood and a new River happening to divide people, random chance plus the continued passing of recessive genes keep things where they’re at.

We’re talking about absolutely massive sample sizes so it works out that you might end up reproducing with someone who has recessive genes you didn’t know about.