Why do girls think they can beat a man up?

Same reason in animes when the short scrawny teen beats up a tall muscular adult. It's a suspension of disbelief. Although some women actually believe that.

Because they know, at least in American culture, men are raised to never hit a woman. They also know they’ll get a “free pass” for doing it. However, if a woman steps up like a man she should be treated like one. Women want equality so give it to them. 

cause they want to beat one up

They beat the sh*t outta men in divorce/family/custody court all across the country every day of the year 

cause they want to for some reason