I believe the number one cause of death for Americans under the age of 30 is traffic accidents.[0] It's around 40,000 deaths per year (in the US alone).
Cars are getting safer, but humans aren't getting better drivers. Individuals humans will get better with experience, but experienced drivers cycle out and new drivers cycle in.
A 16-year-old doesn't get to inherit the experience of the 70-year-old they're replacing. A new self-driving car does inherit the experience of older self-driving cars. They also learn from each other in a way humans can't.
Human drivers also improve at a rate of O(1). Self-driving cars can share their experience perfectly, so they improve at a rate of O(n). I get a day of driving experience per day. Each one of a million self-driving cars on the road will acquire a million days of driving experience. They won't drive traffic accidents down to zero in a week, but in a few years, if we were to deploy them en masse, they could.
[0] Can't actually find CDC data about this anymore. "Accidents" is definitely still #1, but they no longer break out traffic accidents. I know in the past, that was #1. https://www.cdc.gov/nchs/data/dvs/LCWK1_2015.pdf