...why must everything remind me of Perfect Dark these days... *sighs* one more week, one more week...
Back to the OT, *ahem* I really think it is simply a case of both demand and ease of programming. As for demand, since the time where people could more easily play each other online as opposed to split-screen matches, people have been more inclined to want to play with each other rather than a bot. Further, there has been a steady unspoken race between companies to constantly straddle the line between FPS and MMO. As for programming, although this is coming from an amateur (read- never did any programming anything farther than Javascript), I would surmise it is much easier for programmers to create essentially avatars of the characters and have them all have roughly the same attributes (give or take depending on variables such as armor, etc.) and have the player dictate how they themselves act than to create separate identities and quirks for each individual bot. Further, which loops back to the first point, bots, as the name implies, are simply AI running a course and interacting to outside variables. Otherwise they are always able to be predicted once you play with them enough. Actual people however can adapt and change strategies according to the situations that arises. Hell, just play a decent match in TF2 on the Goldrush map to that idea in action.
Just my thoughts. That, and Darksims are the spawn of Hell itself.
That is all.