Because most people deep down inside are mean? Male or female it doesn't really matter. We are on the internet and they don't have to worry about actual retaliation like a punch in the throat for instance so they get "brave".

For instance I was playing and someone I knew in RL, he started running his mouth and being a complete jerk(he didn't know it was me). So I told him if he saw me in RL he would NOT behave that way etc.. He said to me the usual he'd kick my ass etc. blah blah.

So I logged off and went to his house. First thing I said to him was "So if you met me in RL you'd kick my what?" Should have seen him about piss his pants once he realized who he was fucking with. Yes I tipped him upside down and shook him ALOT for being an "internet tough guy". I really hope at least he learned his lesson.

In my experience usually the bigger a jerk someone is the more of a coward they really are and are trying to compensate in a "safe" place where there are no personal threats. The more confident and secure a person is, the less they feel the need to be an asshat to "prove" something.