"because there is no fun being a man"
Seriously tho it's hard make what boys do to be entertaining to girls. Girls already can do what boys do anyway, they can wear men clothes normaly, they can play sport like everyone else, and if they want to hit on girls they have that in yuri already. There are a lot of girls crossdressing as boys tho; all benefits of genderbend, easier to related to than genderswap. So not much niches to fit in.