It's a kind of unknowing arrogance that is created by living in an echo chamber. People who are heavily invested in politics tend to isolate themselves socially from other viewpoints. Especially when it comes to people actually in Washington. How many Democrats work in high level positions in the offices of Republican politicians? How many Republicans for Democrats?

Meanwhile, the corporate media usually focuses on the craziest things about politics for both sides, because crazy people drive clicks, views, etc. I'm going to focus on Republicans to avoid having to repeat myself, but the exact same thing happens to Democrats too.

A Republican politician personally knows hundreds of sane Republicans, and maybe a dozen crazy ones. He might work with a few Democrats on occasion, but they don't come around to Thanksgiving dinner. His opinion is ultimately shaped by the fact that he sees crazy Democrats on a regular basis in the news and at protests. He knows that there are reasonable Democrats out there, but he the ones that stick in his mind are the ones who are screaming at him about how he's a bigot or hates women. Those people represent the average Democrat in his mind, even though they're the extreme.

So what can he think when he meets a quiet, calm, reasonable, and successful citizen? He assumes they must be a Republican. After all, from his personal experience, 90% of the decent people he knows are Republicans. So when he finds out that they're a Democrat, he doesn't think "maybe both sides have normal people and crazy people", he thinks "why would a normal person be in a party with crazy people?"

And the obvious answer is that this decent, honest, hard-working citizen must have been fooled. They must have been tricked! Our politician is downright distressed by the fact that a good person, a natural ally, is voting on the same side as the all the crazy people.

What our politician fails to realize is that people with different upbringings, different perspectives, and different personalities can reach different conclusions from the same facts. The social isolation from normal people who disagree with him leads him to believe that normal people who disagree are simply uninformed. And so he thinks that if these decent people got the right facts, they'd be on his team.

And the reason why they say it out loud is the same reason why liberals wonder out loud why anyone who is poor or gay or a woman would vote for a Republican. It is transparent and obvious that anyone decent would be on their side, so saying that won't cause resentment. It's just a simple statement of fact, and anyone who would be upset is a crazy person who would never agree anyway.