Being stupid is a virtue to stupid people. They embrace it and take pride in it. Waste and irresponsibility is a status symbol. While your grandpa might be proud of buying a Cadillac and keeping it looking like new for years, an idiot is proud that they can buy a Cadillac and crash it. Thus, among idiots, being called stupid is a compliment. However, even idiots know that a smart person is insulting them by calling them stupid, and they are deeply offended, not by the insult but, by the intent to diminish their status.