Thats not the same though. Valve has been constantly upgrading the Source Engine and I think its in its 21th build now(?)
CoD on the same hand hasnt really modified the engine since MW2 (which was 3 years ago).
I dont mind that CoD uses the same engine since 2 now but the game has just been so blatantly stale since the first Modern Warfare. All the games that have come out since 2007 havent had any major changes.
Treyarch are still running IW3 (MW1), they've added what they can to it (independently from IW for whatever reason) but they aren't engine developers. They need to license or buy a new engine soon to keep up.
Respawn are supposed to be releasing a sci-fi game this year too, so blops 2 is going head to head with that, which will probably be running a newer engine but fundamentally be cod in the future from the original cod devs.