Firstly, I just don't think the old image of a bully who beats up a smaller kid is or ever was common. That sort of physical violence is wrong and should be prevented as much as possible. I feel that should go without saying but wanted to make it clear.
These days, bullying has grown to an umbrella term that includes things like teasing. Teasing or making fun of a kid who does gross things is part of a natural reaction from the other kids. Sure you could nudge that kid to stop in a positive way but whose job is that? The other kids? Probably not. The teacher/parents? They have probably tried and failed. So what complacency are you describing? Do you think as a society we can stop kids from being mean to each other?
Teasing is just probing for vulnerabilities with plausible deniability. They're testing you. They're determining your boundaries and testing your response to violations. Officially nothing is happening, they're "just messing with you" and so any reaction seems disproportionate. Such tests often escalate rapidly though until they discover some behavior they can't get away with.
There's a certain point where it goes from teasing to something else entirely
There was a kid I went to high school with who was in a friend group I was also in. There was a point where we started calling him names just as a joke to mess with him. It got to a point where he became emotionally distant, but we kept doing it anyway just bc we didn't think much of it. His personality completely changed. He went from being a happy person to a shell of a person. We were his only friends at the time. He was still distant when we weren't around. He's (apparently) still like this also
I still feel sick to my stomach about that situation to this day, but tl;dr there's a line at which teasing goes from just teasing to something horrible
These days, bullying has grown to an umbrella term that includes things like teasing. Teasing or making fun of a kid who does gross things is part of a natural reaction from the other kids. Sure you could nudge that kid to stop in a positive way but whose job is that? The other kids? Probably not. The teacher/parents? They have probably tried and failed. So what complacency are you describing? Do you think as a society we can stop kids from being mean to each other?