This special kind of stupidity is independent of inherent intelligence. In this case brain power is overpowered by psychological deficiency, namely self-delusion.
This delusion is not genetically produced, but is a result of external influences, notably political, government, media and corporate propaganda intentionally designed to produce delusional beliefs and thinking. Who does this? All sorts of commercial and political interests. The result is a series of biases and blocks, such as cognitive dissonance, to objective facts and information that creates denial about very important conditions affecting the planet, the nation and individuals. People afflicted with this deadly combination appear stupid to those outside their mental ghetto that they gladly inhabit, along with similarly afflicted people.