Something something Dunning-Kruger Effect. Dumb people who know very little about a topic will tend to overestimate their knowledge about said topic. As you gain more knowledge about the topic, the more you realize you don’t know, and the less confident you are about it.

In extreme cases, it ends with the person having Imposter Syndrome. When a person is very knowledgeable and experienced in a certain topic, but believes they aren’t qualified enough to be considered an expert. They feel like an imposter who will inevitably get outed by someone more knowledgeable than they are. So they have a lot of anxiety about speaking on the topic, because they’re afraid it will result in them being outed as an imposter.