I like to fantasize that the creation of futanari ends up linked to space colonization. Here's two scenarios.
>A freak accident creates the first generation of futa. They start reproducing like mad, and since futa can only father futa, within a few generations the majority of humans are futa. Futa have an insanely high reproduction rate, and mankind (closer to futakind at this point) NEEDS to invest in space colonization and exploration in order to have enough space and resources to support the evergrowing population
>We start looking into space colonization and exploration, and realize that we need to vastly increase human numbers to handle all those ventures. We create super-breeders (aka futa) to raise our population. It works, but futa outnumber all other genders even on Earth before we realize it.