Because they have a low self esteem. They have to pick on others to make themselves feel better. Bullying gives them power over others.
Stand up against bullies and it's not so much fun for them.
Been there and done that
Everyone has a conscious but some don't acknowledge it as much as others do.
Upbringing also has a lot to do with how a person treats another. Compassion does not thrive in a lot of people who may feel anger at the world and walk around with nothing but negativity. You can tell a lot by what comes out of some one's mouth and I would say if you don't like what they speak, stay away and find a friend in a kinder speaking person.
Sometimes bullies try to intimidate weaker individuals because it makes them feel they are better than their victims. Maybe there's someone at home who bullies them. If someone does it to them, and they can't do anything about it, they turn their own anger and frustration on others. Some just get a sick satisfaction by intimidating others. And there are some who have no sense of compassion. Maybe nobody showed them what it's like to be sensitive to the feelings of other people. Some people simply have no tact and no feelings for anyone but themselves. It's sad.
Most bullies have family issues that they feel powerless to address at home. So they transmit their angst onto others. It becomes ingrained into their personalities and in a lot of cases if it goes unchecked it can lead them into serious crime.
Humans are inherently good. However, there are persons...as young as little children at that...already manifest signs of cruelty. This is an environmental factor and breeding. When a child grows in a situation where all the child can experience is violence or meanness, there is no reason why the child grows to be the same as what he has grown with. ..and the rest is history! There is an analogous saying: "Hate begets Hate; Love begets Love".
I don't think that bullys realize that they could be part of the fun. I think that they are scared, or shy, to put up a defensive is all they know. Low self esteem problems are usually involved. I have been a bully at times, but it was because I was unsure of myself, and covered it with a defensive show stopper. I had a sever tragedy happen, and it really showed me my place. I did a lot of soul searching and that's what I came up with. Like a wild animal, terrorized, acts out with aggression. Only because he's scared...