COD:BO7's single-player uses dedicated servers. Terrible, yes, but that's as foolproof as it gets when it comes to piracy. They also probably did this to combat all the 'services' offering to unlock camos/cosmetics that you'd normally have to grind for; players would give someone their credentials (probably stolen later and sold to cheaters) and the person on the other side would then use a hacked client to trick the backend into thinking they completed all the challenges. With a server-side campaign, that probably got much more time consuming, I reckon.