I don't doubt that most computer games lack a real AI, but there's a lot more to coding a computer player than just dumbing it down so that it's not impossible to beat. In the games that I've beta-tested the problems are getting the computer players to play the game in any sort of rational manner.
Consider the though processes that you go through when playing in one of these games, for most (non-trivial) games it's usually not something that can be readily translated into an algorithm that a computer player could use. The folks who develop the heuristics for these computer players do some pretty amazing stuff just to get them to play as good as they do. Now if you had a real AI you'd have the additional problems of convincing it that the game was fun and that it should:
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Consider the though processes that you go through when playing in one of these games, for most (non-trivial) games it's usually not something that can be readily translated into an algorithm that a computer player could use. The folks who develop the heuristics for these computer players do some pretty amazing stuff just to get them to play as good as they do. Now if you had a real AI you'd have the additional problems of convincing it that the game was fun and that it should: