I believe they are incapable of doing good enemy AI because if they were capable of doing it, they would have done so already.It's a pretty lame "design" choice to be honest with you.For example, the Halo games have always had good enemy AI especially on the Legendary difficulty settings.That is why I prefer Halo's campaign over COD simply because in Halo SP its actually challenging, unlike that of COD.Besides, I don't play COD for the lamp sp I play it because it has a fun multiplayer component..
If they wanted the talent to write better AI, even though they probably already have it, they would go out and buy it. They're the richest developer around. It's stupid to say they are not a company loaded with talent, and without the resources to get it. I'm sure it's also a very desirable place to work, so I'm sure they don't have to look very hard to get people.
You can compare to Halo as much as you want, and say why you like one over the other, but it has a lot less to do with talent than design.