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Because it's an ongoing story. Everything is tied together, unlike the different "universes" in CoD. If too many developers work on the same game it gets...well, not so great.
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No cod went BO 1,2 & went ghost off story now BO 3 same story.... Man acti prints are all over it.
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And you're forgetting the MW series, all under IW. Treyarch handles Blops with Ghosts being separate because it wasn't tied to any previous series. What I'm say is each studio deals with their own universe/storyline. Treyarch doesn't work on anything IW does and vice versa. If more than one works on a game, you have conflicting interests and such.