Because it may be effective for boys but not girls.
Men have to deal with some form of violence in their lives, or at least the threat of it. Most male encounters has an undercurrent of violence. Offend another male and you might get assaulted.
So when you expose men to violence it's a matter of the world. Like Tyson said, social media made y'all way too comfortable with disrespecting people and not getting punched in the face for it.
If you expose women to violence they will acclimate to it and begin to see it as the norm. That means they'll accept it, from their teachers and eventually their partners.
Its entirely rational to only apply this to boys only.
The mental gymnastics at play here are astounding.
Why would women acclimate to it and see it as the norm while men don't? If anything, exposing men to violence will also make them more violent in their future relationships.
What? Your argument is men should be used to violence, so it is ok to hit boys, but women should not be used to violence, so we shouldn't hit girls?
We hit boys, so it is ok to hit boys, but we don't hit girls, so it isn't ok to hit girls?
That's so very, very wrong.