Back in GT3 I "conquered" 100% of the game and 48 gold licenses, back then I had fun racing against the AI and against myself.
In GT4, and althought I golded every license test and missions races, I didn't feel the need to race against that empty AI, so I just reached 50% of the game or so, the rest of my "GT4 experience" was online, I found it more challenging.
In some ways, I feel GT5P is about the same as GT4 in some aspects, the "races" against the AI are mere time trials with moving obstacles, fake difficulty, because starting last of the grid and in one lap overtake 15 cars sure is pretty difficult, but that's not racing at all. Just driving around an empty track, especially with the new phisycs, is more pleasant than franctically trying to overtake a billion cars moving around like robots.