A large fraction of human drivers are actually all that great. The majority of accidents/deaths are caused by a minority of terrible drivers, or good drivers who found themselves in terrible but rare circumstances. The majority of drivers drive hundreds of thousands of miles without any accidents that were their fault, or even any accidents at all.
In other words, it's probably easy to beat the mean human driver, which is greatly dragged down by a minority of terrible drivers. It's probably very difficult to beat the median human driver, and near impossible to beat the top 20% of human drivers.