Because most people only like it for the multiplayer. I, a gamer that started with the NES, actually finish the single player first before ever jumping into the multiplayer. The mulitplayer is the only reason people refer to COD as a great game.
I know what you mean about them rehashing the same story/gameplay over and over. They are very guilty of that, I stopped buying their games because of it, until MW1 and 2. Their multiplayer is fantastic though and is the ONLY reason for the astounding success they have had.
So it doesn't really matter if they rehash it over and over, because people don't care about the single player anymore with FPS. It is all about multiplayer.
Fact: Earthbound is the greatest game ever made







