It's far quicker, cheaper, and easier to assume someone else's opinion (or just write something off entirely) than forming one's own with first-hand experience. There's also the whole "fitting in with what's perceived as the cool internet crowd" aspect, as well as the "success breeds contempt" thing, which is where a lot of the hating comes in.
Guess CoD and FIFA are the pumpkin spice lattes of the video game world - you don't want to be a basic bitch gamer, do you?