The only challenging thing about AI is that they are wreckers. They don't know how to defend, are relatively predictable and take most corners ridiculously slow, especially when driving in a pack. No it's not "easy" as in I'm sure not everybody can do this, nor do I say this isn't can't be fun, but overtaking a human opponent that you've been battling for several laps is so much more satisfying.
Not only do you need racecraft (being able to race close but not crash / knowing instinctively what your opponent is going to do), skills (to be fast enough to not lose him) and nerves (enough concentration to keep the pressure on), you also have to play mindgames to trick your opponent into a wrong manoeuvre or make him lose his nerves and make an error. The AI is not and will never be comparable to this.