When all examples they ever give for anything consist completely of men.
Have you ever looked into a reddit post on a topic like "Who are the best actors of all time?" Thousands of comments, nearly all male actors. "Worst actors?" Also all men. "Examples of this trope?" Only scenes with men. Stock photos or drawn figures to illustrate their point? You guessed it – all men (unless it's sexist in another way).
For some men we just don't exist as people of equal significance.
Indeed, when I started at my current job there was a Anglophile guy who was really into the Beatles and other "classic rock" groups. Being from an English speaking country he was keen to talk to me, at length, about music.
After a while I noticed that he never mentioned any groups that even had one woman in the lineup, I started asking his opinion about the likes of Janis Joplin and Heart, and he just kind of locked up.
He will now say a frosty hello to me if I prompt him directly, but that's about it ... he has two daughters who don't talk to him apparently.
I've started seeing a few more comics/memes which have women as the representative of a universal experience (rather than, as you say, representing a sexist trope) and it's absurd how happy it makes me.