Because they know they have no punishment for there actions and they can get away with it. They don't have to answer to someone as they would if they were to say it to someones face, and because of that, all manners go out the window. Its sad really.
by your username im guessing you're a girl. im a guy. now most guys on the internet (usually online gaming or anything that isn't social networking) believe that girls shouldn't be online like that (i don't). so usually they're pissed about it or pissed at life so they take out there hate on anyone in front of them or they're just jealous