In the more general case - it's often because of the way we teach. We tend to explain the basics by glossing over the cases where the more trivial assumptions don't hold. Like when we teach "F=m*a", we often don't do a good job with making students understand: "but only for relatively low speeds when we can neglect that mass depends on velocity".  When people get educated further, they'll start to have experiences where they thought they had already understood something, and now learn that reality is more complicated. After a few of those experiences, many will get a feeling for the fact that often things are not as simple as they might first appear. It can help to study rigorous subjects, though that's by no means the only way to be exposed to these experiences.  Of course particularly dumb people tend to avoid education which allows them to avoid these experiences and enables them to remain ignorant - not just about what they don't know, but about the limitations of what they believe to know, as well.