Better is probably a poor word, but consistent at least. Even good drivers may run the speed limit and be "perfectly safe", but the autopilot will always stay at the speed limit and will always make the same choices in a given situation - there's no emotion or influence (alcohol, drugs, bad mood, etc.). In short, they are boring but safe.

Every decision a machine makes benefits from the accumulated organized efforts of hundreds (if not many thousands). In a sense, so too do human decisions, but in practice we can much more quickly improve the decision-making of machines, over time leading to them having the advantage, especially in routinized tasks.

Computers are not apriori better - you can always develop a stupid or buggy algorithm that fails often. But Tesla is working to PROVE (using real-world data) that autopilot is better than a human driver, and IMPROVE it constantly until it is ten times better.