There are a very limited number of people who are just plain stupid, the thing is everyone has areas where they are strong and areas where they are weak, most people recognize strength and weakness so they'll steer towards what they know and are good at and avoid what they aren't so good at. A lot of people have grown up with this belief that they are something special, this leads them to see their perception of reality as the only true one, thus making anybody who falls outside of it a fool in their eyes. I know plenty of people who make poor choices or do something that goes against their interest but they are by no means stupid, it is simply an aspect of their life that they don't operate correctly in. Maybe you are one of those rare people that doesn't judge someone at a glance, you can see this and thus you fail to find prominent examples of stupidity within your life...that's as polite of a way as I can put it.