It's important to note that the default state of people is being dumb. It takes effort, exposure and more qualities/circumstances during a person's early years to not be dumb.

Many people just didn't grow up in an environment that allows that, it's actually a quite specific one that doesn't happen automatically.

And once the early life phase is over, mental plasticity is a lot lower, so very very few people go from "dumb" to "not dumb".

If you want to be less dumb, I can really recommend https://lesswrong.com/rationality