EDIT: I don't have an answer for you, but the answer given above is absurd. Of all of the Call of Duty games released so far (June 2009), only ONE of them has been set in the modern era. ALL of the other games have been set in WWII. People, please don't post answers if you don't have the slightest clue what you're talking about
EDIT: The only time Hitler was presumably ever seen in any CoD game is in Call of Duty: World at War, where a video of him giving a speech to the German citizens is seen among other videos during the briefing of the 5th mission of the campaign (which, technically, is the second mission for the Soviets).
EDIT: There really is no realistic reason for Hitler to make an appearance in any CoD title (in terms of being in a level) because there was no real point at which a player in the typical Allied campaign would come into contact with him. Even at the time Berlin was about to fall/did fall, in 1945, which would be the closest any player would get to him geographically, Hitler was in a secret bunker. He wasn't known to have been there (by the Allies) until the discovery of his body after the war.
It could be because Hitler in a game could be considered controversial. If any is old enough to remember, Wolfenstein 3D had Hitler in it, but they changed his name and appearance so it wasn't controversial.
Having him in the game would be pointless, as he doesn't do anything notable to the COD series, outside of be the leader of Nazi Germany, he has nothing to do with the series. It wouldn't be controversial for the characters to mention him, but it would get attention if he was seen as some sort of a main antagonist like Zakhaev, Makarov or Shepherd were.