Even though the continuing "accident toll" is an ongoing concern, the numbers of cars driven by humans are vast, as are the km/miles travelled, over many years, & in all sorts of road conditions.
The vast majority of the trips taken are safely completed.
On the other hand, there are a handful of self driving vehicles, some of which have already had accidents leading to death.
The argument is "We have operated this vehicle for X number of miles, which is more than the average driver does in Y time, hence, it is safer, statistically."
This reminds me of when germanium transistors first started to be widely used.
We were told "Accelerated life tests & statistics prove that these devices will have lifetimes far in excess of vacuum tubes----- of the order of twenty years!"
When they started to die after a few months to a year, we had to re-evaluate that information!