While my main focus is always on the multiplayer I've finished all campaigns starting from the original "Call of Duty" to Modern Warfare 3. I've yet to even touch Black Ops 2 at this point, I'm saving it for one of those rainy days I guess.
I suspect that most people rate COD based on the context and spectacle rather than the game design. Game design has always been incredibly shallow in COD games.
It seems that the only tick box for COD gameplay is "all hostile A.I. must be shooting the player all at once."
How many military FPS campaigns have you actually played? The entire genre is pretty much the same thing, don't put the blame on CoD. Maybe you should explain exactly how the design is "shallow" compared to other games?
It's a multiplayer game. That means they put all of their efforts there instead of the campaign. I would say 95% of the audience wouldn't buy CoD if it didn't have multiplayer. The campaign isn't supposed to blow us away.