Because they've been consistently bullied at a young age - probably from their own family. They learn quickly that showing empathy or emotion is a sign of "weakness" to those doing the bullying, so they protect themselves by becoming "hard" just like their bullies are. It affects their whole way of thinking from that point on - afraid to show weakness, they become bullies themselves... and the cycle starts anew.

This happens when they're quite young, and changes the entire way their thinking processes work for the rest of their lives. They live in fear of being attacked for being weak, so they eventually bury that fear so deep behind mental walls to keep it from surfacing that they soon forget that's what's actually motivating them deep down. Thinking like a bully becomes so automatic, they no longer can do otherwise without significant therapy.

When they attack you, it's really their own insecurities and a need to feel superior in order to compensate for them that's driving it. Unfortunately, it's a lot like being addicted to drugs - the empowering effect they feel from it dwindles the more they do it, so they often wind up needing to do it even more in an attempt to chase that "high."

But don't bother trying to talk about any of this with them - the truth is SO deeply buried that they no longer know it themselves. It would take a professional spending lots of time getting to know their troubled childhood, and walking them through how it likely affected them to the point where they became the asshole they are today in order to have a prayer of effecting change externally. Otherwise, they'll have to hit some kind of mental brick wall that forces them to look at themselves honestly. As the current US administration capably demonstrates, that unfortunately often never happens for many of them.

As such, don't let them get the better of your self-worth because no matter how bad you are, it's highly probable you're still a better person than they are, or will ever be.