well, not sure if that's the ONLY reason, LeeJend. Another thing that also reduces AI is the game engine itself. i mean, look at older games, like Quake, or even Half-Life. in most of the older engines, the enemies would jus wander back and forth till someone, or something, got in their way. Then, Halo. You throw a nade, they run from it, and, they run from YOU if they so choose. Next game i'm REALLY lookin forward to along with probably a lot more people, Half-Life 2. WOO!! that has some REALLY good AI, and if you think about it, intelligent AI. In the E3 demos, they block a door with tables preventing it from moving, the enemies walk over a bit, break the window, and try shooting you through the window. SOO can't wait for that one.
Another thing that might be making them hold back on AI is the computer it might wind up running on. Not sure if it's much of an issue, but it makes sense in my head at least. If there are a lot of enemies that are trying to figure out what to do in certain situations at the same time, it might even lag your computer drastically. I can play Halo Multiplayer just fine with no real computer lag (unless i'm using my modded maps where i have the shotty shoot 30 plasma's at one shot), but Single Player, when i get into a huge gunfight, it slows down pretty much.