What it really is, is that they’re too stupid to understand simple concepts, but think they’re geniuses. If they, an adult “genius”, don’t understand the concept, then that must mean it is too complex a concept for children, an incorrect concept, or both. The idea of introspection, that maybe they are the idiot and just don’t get it, never even crosses their minds.
Plus just bigotry for bigotry’s sake, of course. There’s plenty of that going around too, unfortunately.