Honest question, who is playing these games for the story? I thought the whole point of this franchise was to run and gun with your pals and 360 no scope. And the campaign was more to help the player learn how to play so they aren't slaughtered in every public lobby.
I played these games for the story. The campaigns used to be pretty solid, with good action and a lot of good moments. I'm not saying they're excellent still, but Modern Warfare had both the nuke and "All Ghillied Up" sequences, Modern Warfare 2 had the up-until-then relatively unexplored front of the United States and the "No Russian" missions, I was genuinely interested in Advanced Warfare's story, and if you want to go really far back, CoD 2 had some excellent missions that really nailed the scope of battles in WWII. A big part of the reason that Titanfall 2's campaign was so highly praised was because they had the talent behind Call of Duty's campaigns working on it. I'm not saying the multiplayer is any small part of the game, but there's at least some subset of people who played it for the campaign.
COD4:MW had an actually good story with incredible use of action set pieces like missiles launching, a nuke going off, and the now legendary AC-130 section that was almost an idle game in the middle of a shooter. The story was so good that it grabbed folks in the industry and after almost 20 years later it is still relevant.
Every other game tried to copy the style but none managed to actually replicate the critical success of the single player shooter. Especially when the buzz was around the genre defining multiplayer.
Call of Duty:Modern Warfare is a stunning game and well worth the effort to get through the story. The rest... retread the same territory as the GOAT.
Mostly young people who like low-standard action media, or people who are nostalgic from playing these when they were young and enjoyed low-standard action media.
Young people? Every young gamer I know has never played the campaign in any Call of Duty game despite playing multiplayer in all of them. The only people who do are older and they're far, far more willing to accept mediocre crap.
In fact these days the young people are all playing Fortnite anyway.