People apparently forget that these kinds of ideas should be "optional" meaning you can play Co-op "if you want to". Its in no way going to hurt the single-player experience.
Except there's a part that ISN'T optional.
The fact that developing and implementing multiplayer/co-op takes resources away from the single player experience. Companies only get a limited budget, a specific number of people and a limited time with which to make a game. Multiplayer development takes a portion of those resources.
Reusing the COD example. The single player experience has got shorter and less fulfilling with each incarnation as more focus is shifted onto the mp side of things. Hasn't hurt sales or the player base, but the games have received some negativity for the impacted single player experience.
That's the sort of thing Bethesda don't want. Todd Howard even said in an interview that they could never find a way to implement mp or co-op without negatively impacting the single player experience and instead, left their focus on making sure the game was enjoyable for a single player.

