I think it’s just an unfortunate part of human nature to enjoy looking down on others. Don’t get me a wrong, a lot of people can get over that, but some people don’t. It’s an easy shortcut to feeling better about oneself that a lot of people fall in to.
When I was younger, I pointed out a friend of mine had trouble making eye contact. It was a shitty thing of me to do because I’ve spent my entire life getting shit on because I avoid eye contact like the plague. I felt immediately bad about doing it and apologized later. I had to really inspect why it was I felt like that was an acceptable thing to do, because it certainly was not.
I find it helpful to point out to people that they inherently understand that it feels bad to have people make fun of you or belittle you, and they need to put themselves in the other persons shoes. Sometimes it’s just that, someone did not even think to make the connection that being treated that way doesn’t feel good, so you shouldn’t do that to other people.