Does the fact that people tend to spend more time on the multiplayer component of a game rather than the single player play any role in this 'not caring about AI'.
I mean, I really did not enjoy playing against AI because they are quite stupid in most cases, and the only way to make a singleplayer become harder does not seem to be to make the AI smarter, but rather have some 'modifiers' against the player.
For example in L4D, the damage you take from a hit by a common zombie on normal difficulty is 10HP (out of 100HP), but on expert the same hit is worth 20HP.
Other things to make it harder would be to spawn in more NPCs and just get the player more busy, or speed up the events in the game (like tetris).
But in all games with a decent multiplayer, it is just _way_ more fun to play against other human beings rather than shooting a bot. Because it actually poses a challenge.
EDIT: typo fix