Because, despite the name of the genre, most people who play MMOs are anti-social, timid, shy, and quiet (straight from the Department of Redundancy Department). Not to mention with the amount of trolls spewing vulgar and hateful comments in most general chats, it's not worth chatting there. My favorite is when I group up to do a group quest with some random person and there's silence the whole time...reminds me of queuing up for random dungeons in WoW where nobody says a word for the entire run.