Because it's sometimes how they truly are.
Also, who said that girls can't like horror movies, guns, martial arts or soccer ?
No one. That's just stereotypes.
Boys and girls are human. We just act as we want to act.
Being a girl is sometimes really a pain in the a## you know. You have to be careful of everything you do not to be labelized in a stupid way.
It's tiring. And a lot of men treat us like fresh meat, like you're walking down the street and they harass you.
So being boyish is also a way to defend themselves.