I have often wondered why but kids can be so cruel! Mean and cruel. And you are right they never consider why a person is like they are, and obviously dont care. There is the story I have heard many times about the student walking home from school and offers to carry another students books. He later finds out that the one boy was contemplating suicide but because he was shown some kindness he did not! It would be nice if kids thought about these things.
Another consideration is some of the biggest bullies are bullies because they are bullied! They are many time abused at home and react by treating others with the only thing they know! Very very sad. And how nice that you are thinking about his. Maybe you can make a change is some of these students!