emeraldrafael said:
Because its popular. For the general answer.
There's always that, but I find that a large chunk of that attitude stems from people who genuinely don't understand what all the fuss is about. Doesn't matter if it's Call Of Duty, Half-Life, Doom, Duke 3D, Fallout 3, Bioshock, or whatever... there is always going to be a small percentage of people who genuinely dislike the game.
If 1% of the customer base dislike the game and you sell 8 million copies (plus whatever the used market is), that's 80,000 people who didn't like it. The more successful the game, the bigger that figure. Call Of Duty is easily the most criticized game here, which isn't surprising considering the franchise regularly sells in excess of 15 million copies.
But Halo fans do seem to be the most sensitive to criticism, seemingly blind to the massive amount of threads started that criticize the Half-Life and CoD franchises.