There's a very specific reason why the single player campaign is so short
Data miners though the source code have found this "Main Campaign" was actually supposed to be DLC for the last Modern Warfare. However Activision just repackaged it and charged full price. That's a total fucking joke.
Activision are a mess a the minute. Player numbers for Diablo, Overwatch, WoW ect are all sinking due to the overly aggressive monetization in those games(£20 for a horse Skin in Diablo 4? Fuck off) and just extremely poor updates seasons ect.
Microsoft have a huge job on their hands to turn it around