For the same reason people think Kent Hovind and Ben Shapiro are smart, it takes time and energy to explain why someone is wrong whereas the claim that oneself is right takes less energy. Compare say a Miniminuteman or Stefan Milo to the average pseudoscience video, the pseudoscience video can throw out 20 claims in the time it takes for someone to explain something that is actually correct.

Also the natural social defenses against this type of shit are effectively bypassed by the ability for idiots to communicate and propagate their ideas. Historically communities had the learned, the experienced, and the wise who could generally call bullshit or otherwise deal with the problem directly, nowadays shaming, beating, and killing are notable less effective.