I don't disagree with you, but I'm going to take it a step further and say selfishness is the root cause. It's not that people are too stupid to understand, it's that they are too selfish to care to learn. You could call that a form of stupidity, but there are plenty of intelligent, educated people who fully understand the validity of the threat and still don't give a fuck because they don't think it personally affects them.
Sure, but a stupid person can act in a way that isn't selfish and help other people. A selfish person who is willing to harm others for personal gain, whether smart or stupid, is a problem for society. Selfish stupid people are really the worst because they will cut the branch we're sharing without realizing that they have just as much to lose.