It’s due to primal reasons. Humans came from a vicious and dangerous environment. Our ancestors were harsh because survival was harsh. The people that lacks social groups are easier to target because they’re not strong enough to stand their ground. It’s how we operate. We like to go after the weak to prove our worth. Other reasons is due to projection, family issues, frustration that leads to projection and people pleasing with the bigger groups.
Bullying is not going to die. It’s human psychology that’s connected to our primal nature.