From the blind, ignorant minds of Activision.
Because other franchises don't do this at all?
It's more like "if it still sells, why fix it?" I know I'll probably get some "master race gamers" yell at me for this comparison, but look at Dota 2.
It plays, according to my friends, almost exactly like the original Dota. Why? Because fans don't want anything else. They don't want a drastic change. In their eyes, "if it ain't broke, don't fix it".
For Activision, they still are able to sell millions upon millions of copies of CoD (on launch, mind you), which shows that their fans feel the same. Even when they had the fierce competition in Battlefield 3, which some thought was gonna be a "CoD killer", MW3 still broke sales records and outsold BF3.
If you're gonna talk about "OH IT'S $50 WORTH OF DLC", I'll point you to Guild Wars. They only made money on selling new $50 campaigns every year. I mean the campaigns introduced entirely new stories, new classes, new skills, and many other improvements, but it was still $50 for a game that ran on the same engine and had the same basic mechanics.
I'm not even a big CoD fan but if you really think every CoD game is identical then you're really quite wrong. Similar? Oh yes. But they add new stuff, it's not like every franchise does drastic transitions with every game.