They do it because they are insecure about themselves and very probably jealous of your good nature. Don't let them get to you. Rise above it and believe in yourself. By self-harming you are punishing yourself for their inadequacies. Be better than that and trust in yourself. You don't need to be around losers.
because they are sick people, and all they want is to harm others, and when you cut yourself. The answer to those bullies is not harming yourself there are other solutions.
bcz u listen to them and that make them feel to do it more n more ...
because you're so beautiful.
They do it because they can. There's no sophisticated or theological answer. There's no universe issue that requires this. It's not a test it's just abuse. They are abusing you because they can. It makes them feel powerful and they like it.
Much like bullying people, some people sure find pleasure in the suffering from others like that, particularly knowing they cause pain. Only way some find value or like they mean something it seems like, pretty pathetic really.
Many people suck. You will learn that more in life. They like to be mean. They are immature, off-center, sadists, big egos, neurotic, etc. Keep space from them if possible.