accidents are not equally distributed across humans. more serious accidents will be caused by people who are habitually doing things that are unsafe but, for various reasons, most places lack effective ways to stop these people driving so they keep causing accidents.

the metric is not some nebulous aspect of skill but the bottom decile of human drivers causing accidents. it is not difficult to believe that an AI can drive better than this group, it is not a high bar, below the 10th percentile are people who should not be driving but cause most of the accidents.