It depends on if the developers are happy with the engine they are using, and what can be done with it I suppose... and also if they literally started from scratch or just reworked the previous engine from the ground up. I can't find any actual substantial information on the AnvilNext engine, though(other than the cookie-cutter press information that every website seems to have copy/pasted), so I can't say for certain whether they truly made it from scratch or if it's just the next iteration of their current engine. edit: Okay seems the only source I could find: http://www.gameinformer.com/b/featur...il-engine.aspx implies it is made from the ground up.

However rarely do developers feel the need to do that, and rarely do their games call upon such drastic measures. What would improve from Call of Duty with a new engine? As far as my opinion goes, I think Call of Duty has some of the best lighting/shadow implementation on the console games(at least as far as multi-platform games go) and the graphics are solid enough; but engines don't define graphics. Their main purpose has always been to constantly have 60fps gameplay, and their engine offers that(you'd be surprised how many games DON'T run at 60fps, but instead 30fps or so). The bullet physics and such defined the current fps genre and are still holding strong, so no need to make changes for that... I can't see a single reason why they'd want a new engine. MW2 made substantial upgrades to their engine, and I think Black Ops 2 is doing the same thing. Standard stuff here... I just don't see a need to criticize the engine.

And yes, creating engines is hard work, and more importantly fairly time consuming. Remember that engines control EVERYTHING in the game and are basically the building blocks. So when you start an engine from scratch you're essentially starting with a blank file and doing EVERYTHING in the game all over. This includes lighting/shadows, animation support, physics, NPC AI and... EVERYTHING. You have to code all of that all over again. This is why many games don't develop their own engines and instead use commercial engines. For example Fallout 3 was made using the engines from the Elder Scroll games, which in turn were apparently using a commercial engine called Gamebryo. Many games use the physics engine Havok, instead of creating their own physics.