We do. In some of the set in the same universe novels but not written by Asimov there are references to Brain Fever. A disease that virtually 100% of humans get at least once that makes them dumb for life. All advancement in galactic culture comes from the like 1 out of a million people who were immune.
That would account for Dune. Dune only makes sense if you assume that everyone is stupid and living in a hazy of drugged religious fantasy. Ffs the main power of their space witches is to use a sexy voice. Which everyone knows about! Just put in earplugs or jerk off prior or get gay guys or use deaf people or get straight women before dealing with one. Thousands of years of eugenics defeated by 30 cents of earplugs. Dune everybody!