Because with waymos anything learned by one is immediately available to all of them. Therefore you have lots of miles but effectively one driver. Now if you take human drivers and pick only the best, how do you numbers look then? Many people have driven lifetimes and not been in an accident or not in an accident that was their fault or reasonable predictable and avoidable by them.
Driving needs to be considered a lot more serious of a task than it currently is. There are many driving errors that are the primary cause of accidents but those people are never forced to get extended drivers education. Self driving cars are nice but if the goal is to prevent accidents it's a bit like requiring everyone to order prepared food to prevent injury while cooking.
At some point we need to get back to shaming those who are bad at something and requiring that they improve their skills rather that restricting everyone else's freedoms.
The numbers may look different, but does it matter, though? This is a benchmark, not a competition. Since we can't possibly get every human driver to the level of the best drivers, it's still a win for Waymos.