Why are humans stupid?

Because they're too lazy to use their special something called a 'brain'.

When people face an uncertain situation, they don’t carefully evaluate the information or look up relevant statistics. Instead, their decisions depend on a long list of mental shortcuts, which often lead them to make foolish decisions. These shortcuts aren’t a faster way of problem solving; they're just looking for the quick fix. People need to remember that life is not a quick fix, but an opportunity for all of us to learn from each other.