I haven't played GT that much (And probably won't anymore.. one memory card of mine just died with my GT savegame and also savegames of Tomb Raider IV with more than 14 hours of play
), but yes, I've noticed this too.
Yes, I know in ATR the cars are programmed to shortcut in specific points, or in Nitro they are programmed to make mistakes to look like humans. In overhead racing games, the AI basically follows a racing line, or a group of racing lines, choosing the best (or random) line for that moment.
Yet, you can see the difference between the AI in ATR and in Micro Machines. I think it still will take a LONG time 'til we see a human-like A.I. in a video-game, but what we have today is pretty good IMO.