In brief- why do bystanders not address abusive sexual harassment they witness?

I live in a major metro and sometimes use the subway to commute. Yesterday I saw two completely separate and noisy incidences of catcalling/heckling, both directed to women from male catcallers. (I specify gender because I don't want to 'assume' a catcaller is male- but in these instances they were.) The second incident was completely over the top, very explicit, in a somewhat enclosed setting so not only was it unpleasant to hear, it appeared rather threatening. The woman was on a packed escalator full of people and a heckler walking alongside the escalator down the stairs.

A good 20 people each time likely heard/noticed each of these incidents. Nobody said anything. Do the people witnessing this take away an unremarked symptom of 'rape culture' when they see this?

The second time I address the guy and said "Hey, shut your mouth, you're making the rest of us look bad" and people smiled and laughed and the guy had some further obscenities to me and left.

I really don't understand why people, male or female, don't speak up when a fellow citizen is shouting gross things in front of a sea of people, including kids. If we ignore it, we're condoning it, right? I do not see this at all as white knight behaviour to say to the guy 'hey, behave yourself, you're in public' because the same should be said to any person shouting vulgarity in public.

I know full well I'm not 'one of the good ones' because I don't act like a boor. I'm one of the normal ones, the overwhelming majority of men like me that just want to get to work, get home, take care of my shit and live a decent life.

My daughter is a very young teen and is already getting this garbage shouted at her on the street. (I have no idea if similar gets shouted at teen boys, I am sure it does at times, just not my personal experience.) I do not understand why a group of decent human beings would let any of their friends shout trash and not speak up about it.

When we let the rare outliers shout abuse unchecked, and we stand there and say nothing, this is contributing to the idea that all men are harassers.