@chrisb555,
I think for at least some people, it's a lack of knowledge of anything outside their small town or ideological bubble.
In 1979, I was a freshman in college. One of my suitemates told me that I was the first Jew she had ever seen in person.
This girl wasn't from the sticks. She was from just outside of Providence, Rhode Island --- and all of Rhode Island is maybe an hour to an hour and a half from the borders of Connecticut and Massachusetts. She went to a decent high school and was the salutatorian of her class.
We are still friends, BTW. She was genuinely curious about me and my background.
In the late 90s I worked as an auditor for a large insurance company. I handled the south, and traveled with a few separate CPAs. One was a young woman who had gone to Liberty University (Jerry Falwell's school). Again, I was the first Jewish person she had ever met. She didn't really understand the difference between Jews and Christians until she asked me what Jesus meant to "my people". I think she finally got it when I told her it was the same as what Muhammad meant to her.
As late as 1999, in rural Georgia, we would see tee shirts for sale at the Cracker Barrel that said, "Jew him down!"
Even now, I know people who have been asked if we had our horns removed. And I have seen plenty of antisemitism on A2K as well.
And that's just good old fashioned casual antisemitism, against a group of people who often are a kind of invisible minority. If I say nothing, and don't wear a Star of David, people often don't know. In 2018, the CEO where I work asked me how my Easter was. I told him it was Passover.
To this day, in a company of ~60 or so people, only two of us are Jewish, and there's one Muslim guy. Christmas is a day off. Yom Kippur is not (in all fairness, we have unlimited PTO, so it doesn't truly matter these days).
On the other hand, there's a larger proportion of people of color at my job than in the general population of the US. The company is committed to DEI (I wrote the DEI statement). So, there's an effort being made.
The point I guess I am kinda dancing around is that even educated people who are making an effort have an undercurrent of not necessarily hate, but definitely ignorance.
This is the case even in as connected a world as we live in now. A person can hit Google to learn that Jews aren't born with horns-- yet there are plenty of people who don't bother to find out. They aren't even intellectually curious. Same with crime statistics or demographics or anything else where people make assumptions based on race, religion, gender, etc.
Yet a lot of people don't test their assumptions. And they don't stop self-segregating. Why is that?