The thing is, not that many people stick around a CoD studio for very long. It has improved now that they alternate releases, but when you operate studios that have "pre crunch" in order to make linear corridor shooters then you accept that a certain percentage of your talent just won't stick around for very long.
They did change the bonus structure after the fiasco that lead to Respawn forming to force people to stay longer.
Like the local progression in LA for awhile was work for work for hire studio, go to Activision and work on a CoD game or go work at EALA, then go to Insomniac/ND/Sony Santa Monica, then burn out or something.
Riot and Blizzard gobble up a lot of top tier talent now. Since most of their offerings aren't on normal holiday release schedules they can offer non crunch based work environments.
There's also the sheer scope change. Just for the "developer" studio, Call of Duty has about 25 people credited and no additional outside contractors (outside of music/sound), 2 which added console development goes up to ~45, 4:MW comes in around 75 and adds a few contractors...by the time you get to Ghosts there's 300 people credited that aren't listed under the Activision headers with easily over a hundred for just the main developers and then as many contractors plus other Activision owned developers like Raven/Neversoft/etc.
Ghosts shares 467 credits with MW3 which shares only 140 credits with MW2. MW2 shares 143 credits with MW which shares 90 credits with 2. Only 70 credits are shared between 2 and the original game. That includes all the Activision people.
Only 44 people worked on all the Infinity Ward credited games from Call of Duty to MW3, also including Activision listed staff.
Infinite Warfare is comparatively even more insane, it's easily got a thousand people credited. It shares 611 with Ghosts, 299 with MW3.
Mass Effect 1 and 3 share 146 credits but 2 and 3 share 314 credits. 3 has some 750 people credited.
I agree at some point we no longer have the ability to attribute them to a developer as a name. And that point probably actually was ten years ago or more. Sooner depending on if the company is French...
spoiler (click to show/hide)
Assassin's Creed, number of people shared:
I -> II: 358 people
II -> Brotherhood: 851 people
Brotherhood -> Revelations: 848 people
Revelations -> III: 1153 people
III -> IV: 1482 people
IV -> Unity: 1489 people
I -> Syndicate: 191 people, one of whom is Yves
