Because people feel like it isn't fair when you don't sit there and get hit by their own stuff. As a Kitana player, I can rarely win on ranked without my opponent rage quitting. I recently played a Sonya who did nothing but attempt to outzone me (which failed) and would only do d3/4 when I came in close. After the ragequit I got a message saying "I don't play with spammers", to which I replied "sorry for not stopping to eat your attacks". I then got more messages from this person claiming that he is on such a higher level than me.
PunisherAlive, just sit back and laugh at them, and remember to enjoy the knowledge that you're the better player.