Fuck me with a stick, but I don't know why you're getting so many annoyed responses, OP. I instantly understood what you meant and I agree with you completely. In fact, I've been thinking the same thing for a while.
My theory on why people use this line is this: They are so completely incapable of dealing with an opinion that differs too greatly from their own, that in their mind they immediately think that the person against them is dumber than dirt. "Why would he believe this, if he wasn't completely retarded. He must be."
And so, you get "Go back to playing CoD, this game is clearly not for you", or "You must like CoD" or some variation thereof. As people have already correctly pointed out, this is the same as bringing up Twilight, or Justin Bieber or Transformers.
Personally, I find it helpful since if those statements are followed by a long-ass post I know to prepare myself for some annoying elitism, or to roll my eyes and skip reading them altogether.