AI cars go fairly slow when in a pack because they cannot know the skill of the other drivers. A human driver approaching T1 will be more cautious if he knows he is racing a couple of noobs; he'll drive closer to his limit when he is among experienced racers. The AI has no such notion, and will only react to things that happen. This effect gets stronger with the size of the pack: car #2 slows down a bit in response to car #1, then car #3 must slow down some more to avoid bumping into #2. Car #4 brakes even harder, etc.
The same holds for taking curbs (point 6). Human drivers can do this because they know how the car will behave. They anticipate the reaction, and can start countersteering before a wheel hits the curb. AIs can only respond. And on the more powerful cars any response will be too late.
BTW, in some games the AI can "cheat" the physics (e.g. by getting extra grip), to compensate for their inferior driving skills.