These men feel prevalent because the decent men aren't shouting anything out of windows.
Think of the number of men you encounter in daily life who don't do this. The millions of men who aren't posting shit about women online.
The shit men force themselves into women's worlds by making a nuisance of themselves.
I had a boyfriend years ago who called me fat. I had a 24 inch waist and was 8 stone. And it wasn't just him. Random men in the street would tell me I was fat or ugly when I wasn't either.
They do it because some men genuinely believe that women only exist for their pleasure. They have no concept of women having inner worlds, lives, dreams or ambitions beyond attracting men and appealing to men. So when they don't find a woman attractive personally, they feel affronted that that woman has existed in their field of vision without being of any benefit to them.
Equally, they do it to attractive women who they conclude (through their own insecurities or inadequacies) wouldn't be interested in them. Then they are affronted that that woman has existed in their field of vision without considering them as a sexual partner even when it's a random woman just living her life who has passed them by in the street. So they decide to bring her down a peg or two by telling her she's fat/ugly and no one would want to shag her anyway.
These are the men you see posting online. Usually criticising a woman for her appearance (too thin, too fat, too muscular, too toned) or her achievements and saying things like, "Men don't find abs attractive," or, "Men aren't interested in women with PhDs."
Like every time women do anything, their only consideration should be, "Will men find this attractive?"
In reality, the men who do this are telling you everything about their own insecurities and sense of inadequacy and not saying anything at all about you.