Chess and go are closed systems. And humans can get better at driving, especially if they have better tools. And really, if you want to maximize safety, you can just make cars safer and slower. Then it won't even matter how good of a driver you are.
But what computers are good at is not getting drunk, tired, and distracted.
You have hit the mark. And it isn't just exceptional circumstances; there is a skill and expectation distribution amongst human drivers that can be eliminated here.
We don't really care about the best human drivers. The challenge is preventing the worst human drivers from sitting behind the wheel and eliminating edge cases where an otherwise competent driver is not fit to drive due to some personal circumstance.
To piggyback off what you said -- "best" is much harder to define in driving compared to chess and Go, because those games have a much more defined victory condition than does normal driving. How do you "win" at (non-competitive) driving? A driver with a 0% accident rate might not necessarily be better if they drove at a speed of 0.5 mph.
But what computers are good at is not getting drunk, tired, and distracted.