Let me guess (I read the book years ago).

Humans, in any situation, invent something to do, simply because "doing nothing" is not an option. A stupid social interaction is usually preferable to no social interaction. On the other hand, an intimate interaction increases the risk of being hurt, so with strangers people prefer rituals. Ritual provides some small social interaction at almost zero risk.

If I understand it correctly, Salemicus suggests that catcalling is simply a ritual. It is more than nothing. It is less than a personalized message. It is what other people (of the same social group) in the same situation would do.

Why exactly this ritual instead of something else? Dunno. Tradition. You usually don't invent rituals, you inherit them from your ancestors. Somewhere in the past, there was some reason. Maybe a good reason, maybe a random incident. Doesn't matter today. This is the ritual we have. This is what we do when we want to do something, but not something personal.

And of course anyone who gives [the reason] will be the designated patsy and thereby validate the feelings of moral superiority the coalition has been endowing itself with.

is there a way you can summarize this reason?

As I already stated in the original post - no!

Besides, you don't need to read the book to know the reason. It's the obvious reason. I simply referred to that book because it explains the entire social dynamic around it.