This is true, though the AI does have great pathfinding, it is heavilly scripted to whatever environment it is in. That's part of the reason that there have never been any bots in multiplayer, because rigging the AI to work for all the game modes for all the maps would be very difficult with the AI being so object reliant/semi scripted. Not that it couldn't be done, it would just be difficult and time consuming to make a reality.
The AI in Killzone, however butchured, is a more dynamic approach to AI that I think shows promise but needs a) more powerful hardware and b) more polish. You can read about Killzone's AI here:
http://www.cgf-ai.com/docs/straatman_remco_killzone_ai.pdf
Though, personally, I don't mind the AI of Halo, they're predictable but never stupid and typically a bit of a handful on the higher difficulties.
With Killzone we'll just say that he AI can somehow come across as smart and stupid at the same time. Part of the problem, imo, is that the units are tailored for what appears to be realism, yet enemies and allies can take so many bullets (not to mention that your squad amtes can't die).
EDIT: overall I would say that Halo does have the best AI out there for a FPS. They are predictable, but they're great fun and a challenge all the same.