Calling people stupid is simply a performance for the fellow travelers in your audience. It’s a way that we can all come together and agree that we don’t have to engage with some argument, because the person making it is a bovine lackwit without the basic intellectual equipment to come in out of the rain. So the first message it sends — “don’t listen to opposing arguments” — is a stupid message that is hardly going to make anyone smarter.
The second message it sends is even worse: “If he’s stupid, then we, who disagree with him, are the opposite of stupid, and can rest steady in the assurance of our cognitive superiority.” Feeding your own arrogance is an expansive, satisfying feeling. It is also the feeling of you getting stupider.