Why would any girl want to be a boy? That’s just nasty!
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.