Again, I'm not giving you my opinion. I'm directly quoting from a legal brief, filed by the Infinity Ward CCO and CTO. I didn't work on the game, and so I have no idea what pressures the team was under. For what it's worth, the wikipedia entry for the game says they developed a new iteration of the engine for MW2, "which…
Seriously: From wikipedia: [History Before the formation of Activision, software for video game consoles were published exclusively by makers of the systems for which the games were designed. For example, Atari was the only publisher of games for the Atari 2600. This was particularly galling to the developers of the games,…