Maybe it’s because girls can be molested by their mother’s sex partner. (So can boys, but it’s less likely.)
Maybe it’s because society wants girls to be “sexy,” that is, to stimulate male lust, even before they reach puberty. It might be easier in the school environment to live as a “trans boy” than as a girl who doesn’t want to participate in the sexual marketplace.
I know for a fact that there are middle school, high school, and college girls/women who put on a “butch” persona because they don’t want to invest in becoming “hot, sexy things”. In other words, they don’t want the “role” of being sexual teases to the boys — which is what this so-called “culture” relentlessly pressures girls to be.