because like you said, their stupid. They don't know any better. But if it is a normal person, they are just insecure and want to hurt people. Just don't pay attion and dont show emotion! They know they can dig inside of you and hurt you. Be strong and evently they will stop.
Some people also have nothing to lose. Their life is miserable for any number if reasons. Some no fault of their own. They hurt others to help them feel better about themselves. Or know that if they bring others down they aren't alone. And some just do not care for others. Just themselves