Because it gets drilled into most boy’s heads from a very young age that you are never to strike a girl, not even in self defense. Violence by women against men gets a pass to a degree that’s almost ridiculous, including sexual violence - though with sexual violence it’s baked into the systems and discussions around it to the point that I can actually find you quote from people involved with researching or dealing with sexual violence being pretty explicit about the double standard and treating it as obvious.