I can't help but wonder is it partly because:

Young women feel they are judged more for how they fit in socially while young men feel they are judged more for what they do?

OK, I admit, this is pretty vague, but here goes. Young men often feel confident of their importance so long as they have a defined place where they are judged superior (or best: irreplaceable), whereas young women often seem to be more interested in how they fit in socially than their own abilities. Thus Ringo could feel confident of his importance and his social place because drums were important and Ringo played drums better than Paul, John, or George. And even before that, he could hone his skills with drums because he felt fewer social pressures to conform, and much MORE social pressure to excel at SOMETHING. In some studies, young women are shown to do worse when in mixed classrooms, probably because they don't want to take too much of the spotlight from young men seeking to excel. Young men seldom hesitate to rock the social boat by trying their best in their field of expertise. Now, my guess is that while it's possible to become a really good solist practicing mostly on one's own, away from social pressures, perhaps learning to be a really good band member can only be learned within a band? And for teenagers, that seems to be done more easily by young men.

So even before child-rearing years, young women have to try harder to specialize in music, especially small-group music, than do young men.

And yep, I'm glad that there are more all-girl groups these days than in the past.

Just a totally irrelevant question: are there more all-girl groups in Japan? Are they talented?