Is there something very weird and special in that problem? Because otherwise, we've surpassed human ability of handling dynamic and fast-changing systems like, 50+ years ago? The whole field of control theory deals with stuff like that, and I'm convinced a decent enough feedback controller will beat human intuition any time in problems like these.
I'm speculating a lot, but if I had to say why driving (fast) on dirt is substantively more complex than driving on tarmac for an AI, it's because road conditions change much more rapidly on dirt. You're expecting to be constantly in various states of intentional traction loss (sliding), including having a few or all wheels off the ground with some frequency (jumps).
If you're driving slowly enough to maintain proper traction, then there really isn't an issue. But if you're rallying down an Argentinian mountain trail (take a look at some WRC on-board recordings on Youtube for an idea), you'll have an extremely challenging planning problem.
I don't think it's insurmountable for AI, it's just that racing over dirt has a lot of nuances especially to car handling that you never have to deal with on tarmac because good tarmac racing involves basically 0 traction loss and smooth, static surfaces. Not that tarmac racing is easier... where the track is simpler, you make up for it in sheer speed and the complexity of having to share a track.
Secondly at least for human drivers, the sliding keeps you in control. Driving very fast on loose surfaces is bound to make your vehicle slide around. By initiating these yourself, you are in control - you can feel the amount of grip available, so by oscillating over and under the limit you can keep your speed high while still being in control. This last aspect of it is by far the most difficult and interesting aspect of designing such an algorithm in my opinion. This is where the "Scandinavian flick" for instance comes into play. But I will have to disagree with a commenter below that said this was done at 4:34 - that is more a correction of the exit of the last turn than a true Scandinavian flick. Solving the oscillating aspect of dirt driving through driver feel is something I am having a very hard time believing can be solved by algorithms. Driving around on hard surfaces where the friction is somewhat known I could see algorithms come close to humans, but on loose surfaces it seems quite a long way into the future if ever.