There are so many ways to reply to this but one would be that they weren't raised right. They don't have the conscious to be careful of what they say or how they make someone feel, some subconsciously putting people down while other intentionally doing it. It might be that people put them down and when they feel bigger or stronger than someone else, they take their hatred of that person to someone weaker because they are too weak to face their own problems. Maybe another thing i could say is that while some people do this, not everyone is perfect. Many, many things to say about this topic but the main thing is, people are fake. You can be poor for your whole life and they mistreat you but once you get that money and that job and those connections, they hang around you like leeches. Surround yourself with people who see what a gem you are and not the people who get off on hurting others. You could mean poop in one place but a million in another. Find that place and it wont matter if someone says your are poor or your not smart, they know your better than them and they will try to bring you down.
It could be the way they are raised or that they are struggling with something themselves
