You can put a lot of effort into something that looks like it was thrown together by the c-team in a month.
Battlefield 3 and hardline both had that problem (Didn't even look at 4). When the campaigns miss the point of why people play those games by such a wide margin, it doesn't matter how much effort was put in - it feels inconsistent with the rest of the package.
And something like Titanfall, where there was evidently money and thought put into the campaign still felt tacked on. It was stapling a mission structure on random multiplayer maps, where swapping sides, voiceover, and the general structure were strange compared to the rest of the game. Unlike old Call of Duty entries or Bad Company, there was no reason to even try the campaign - it felt like an afterthought.
Consistency, effort, and length matter more to perceptions of "tacked on" game modes. The original CoDs were relatively short, but had great pacing and structure to pull the player through. Too much attention is being given to "ridiculous globe trotting adventure" and not why people liked the old campaigns. It's why CoD4/MW2 are remembered fondly, while MW3 is ignored - there's a reason people cared about a guy named Soap. All the skilled writers, voice actors, and set pieces don't mean anything if the effort doesn't show.