I think maybe romantic attachment is more relevant than agency? Because while it is completely possible to write a love interest out of a ship fic, it's still harder than writing ship fic for someone who is unattached or casually dating. And female characters tend to be romantically attached quite a lot, and more "seriously" than many male ones.

IDK, I do find it very hard to write femslash for the most part, in the sense that I have to go out and actively look for slashable characters to find them, since movies/shows tend to not want too many women in a room together. None of that is an excuse to avoid het or gen, though, since that only requires one woman, and most movies/shows can manage that.