This is a shallow question. For one, it is too general and doesn't really have a straightforward answer that applies to all men. Two, you cannot possibly have considered the things each and every one of these men have gone through. The traumas, the sadness, the disapproval, etc. For example, it is not putting the idea that their father was possibly a daunting figure that showed them that it was a queer thing to do to share feelings, be in love, and not be full of yourself into consideration. Insecurities are a real thing, and this question could be paralleled by the other extremely shallow question: "Why are women so bitchy?" Again, no one has the right to claim that they know every woman's problems in the whole world. So, it is best to keep these horribly unthought and usually angrily brewed thoughts to yourself, and rather be introspective and try to understand yourself and why men bother you before you call them all stupid. Because that is sexism.
Men are stupid, mainly because they don't always think. They can sometimes be at fault for not using their brains full potential and thinking below their belts. Have faith though, there are smart men out there, they are just few and far between!