Note that the statistics we have to work with are relatively terrible. For example: Waymo's favorite statistic is "x miles driven", which is a terrible/useless statistic, because it treats all miles equally, and fails to note that the most complex driving is often short distances (intersections, merges, etc.) and doesn't account for the fact that most of those miles were repeatedly driving on a very small number of roads. But it looks good on marketing copy because it's a big number.

Additionally, our self-driving car statistics we tend to see today also tries to ignore the presence of test drivers and how frequently they intervene. As long as they can intervene, the safety record of self-driving cars is being inflated by the fact that there's a second decisionmaker in the vehicle.

EDIT: Oh, and human driving statistics are also bad because a lot of accidents don't even get reported, and then when they do get reported, a lot of it is through different insurance companies, that's before we get into nobody centrally tracking "miles driven", and that's why most statistics for human driving safety are more or less just an educated guess.