There are a host of obvious reasons why even a "simple" L4 car will beat a human hands down:
* Reaction time.
* 360 awareness / visibility.
* Lack of exaggerated "Human Reflex" to surprising events (i.e. swerving violently to avoid a dog, and hitting other vehicles)
* Keeping perfect space around the vehicle for safe stopping at all times. (don't tailgate, don't get rear-ended)
* Assuming a LIDAR-based system, virtually no difference in day vs. night vision. No "sun in your eyes" or "road glare".
* No fatigue related accidents.
* No DUI and related prescription drug accidents.
* No distraction-based accidents. (kids, cellphone, food)
* No "road rage" based accidents.
I could go on, but this should drive the point home pretty clearly, IMHO.