Feel the same, age plus the fact that I think maybe everyone has a cap for the amount of time they can spend playing Call of Duty style MP games and I put in so many hours to MW1/2 and WaW that I only manage a few hours of the recent games before I just feel bored.
Not sure what it is but CoD also seems to annoy me more than the likes of Destiny, Overwatch, etc as unfair deaths seem more common and the player base is pretty unpleasant. Most of my online time now is either co-op stuff like Destiny raids or Nioh as I find I enjoy working together a lot more than fighting each other but that is probably a sign I'm getting old at 37.