The oversimplification is that when men are writing they believe they are talking about humanity as a whole, but are undervaluing or even excluding women's perspectives. Women, on the other hand, quite consciously include men in their writing. It would seem at first glance that women are therefore writing more comprehensively. Yet it doesn't seem to work that way, does it? Or not often. This is because when women write they are frequently looking at the specific topics of male/female interactions, of a woman negotiating a world dominated by men, or women observing themselves in their romances.
The relations between men and women are one of the largest topics of humanity, yes. But there are a thousand other topics that women are much less likely to address. Male comedians will talk about women and sex, but they will riff on many other subjects. Women joke about men, or about sex much more frequently. I sometimes have the impression that it is other women who push women writers back into the boxes of such topics. But I am not a woman writer, so I may not see this clearly.
I noted in my comment under James's discussion about the Great American Novel that Willa Cather would be an exception. Even though marriage and male-female relationships are a good deal of her material for her novels, she is writing about settling the West, and about American attitudes and opportunities and dreams. I'm collecting other exceptions if you think of them. Black and other ethnic female writers sometimes focus on that aspect of negotiating with the rest of the world, but that seems like a parallel track.
Yet there is another large category of women writers who are exceptions to this, and they are the writers of children's books. Laura Ingalls Wilder is writing the American frontier experience. There is no effort to make the protagonist generic or seem anything but a girl, but the perspective is meant to represent us all.
Not really a digression, because it bears on the main topic, though from another angle: I note that "us all" means "both males and females, but probably only whites" in this context. It is very hard to put any kind of real character out there that does not make some group as a whole feel they are being left out of the discussion. Bill Cosby's early comedy succeeded because everyone - well, all males especially - could recognise their childhood in his descriptions. Jewish comedians could do Jewish mother jokes because non-Jews saw a lot of similarity in kind, if not in degree. When writers and other creators attempt to do Everyman, it nearly always is indeed a man. I will not comment on what that says about our culture and psychology here, I only note that it is so. Everywomen are rare.
Back on topic: Anne Shirley is clearly a girl in all her episodes, and it is well-known that boys don't tend to read books with girl protagonists. Yet her story of abandonment, failures, coming-of-age, and success is about humans in general, not just girls. One series of Madeline L'Engle's books is read with pleasure by boys, girls, and adults, and is not race-restricted. But her "Austins" series is very much girl-coming-of-age, and it would be an unusual boy who would take to it.
The Young Adult novels, and even YA nonfiction, are far less often accessible to both sexes, precisely because the gender differences, and the navigation of one with the other and the pursuit of identity are the point. That is also a large issue in all of humanity, but it is still necessarily narrow and exclusive. It doesn't include cabbages and kings, or why the sea is boiling hot, or whether pigs have wings. To choose that genre includes a great deal of pressure to have topics chosen for you.
Picture books and those aimed at the youngest children are even better for including both males and females, as well as all ethnic groups. You can zip just about anyone into the illustration and it's the same story. And even farther down that line, animal characters can be very generically human, which is part of why they are a useful device. However, there is now a limitation on that. While animals can be sexless in literature, once one starts to go near the topic of sex at all, they are going to be quite binary. A trans hedgehog is clearly a sermon in the making, not something that is going to flow naturally out of the data. So it may not be surprising the women writers gravitate to writing children's books and animal books. That children are, and certainly were considered one of the special spheres of women is likely the main reason for the focus, not only because of interest but because of easier cultural acceptance of the work, yet the greater artistic freedom, of portraying worlds of girls who nonetheless represent both sexes is also a draw.
Thinking of picture books put me in mind of Susan Jeffer's illustrated Robert Frost "Stopping By Woods" and it is quite true that it could easily a female, or a person of either sex from any race or group. If we ask why this is, we see that the generic human being a a white male is double-edged. Yes, it proclaims "majority culture." But it is also rather undecorated, a vague clay sculpture which the reader can decorate as he likes. Even the term"everyman" is rather indistinct, like an undecorated Christmas tree. Even a male protagonist written by a woman can be that. At least I think so. There may be hundreds of examples of Everymen written by women, but I'm not thinking of them.