The reason they investigate every airplane crash for a root cause is not necessarily because the number of deaths tilts the statistics, it's because they are highly complicated mechanical and electrical machines where there is always the chance for a systematic fault that will doom many further planes if not found and rectified.
The same of course applies to self-driving cars; they are literally cars driven by software, of course you need to do a root cause investigation every time to rule out that it's not a bug in the software that will kill another person (and many after) when the next car happens to go down that rare branch of the system.
It's embarrassing to see that the people that call themselves engineers at these companies have not held their work to this standard, and are instead publishing glossy brochures making whacky statistical arguments.
Why do you think they don't do root cause investigations?
I've personally read through the root cause reports for most of the notable AV accidents. They're not always quite as intensive as aerospace, but I'd be hard pressed to describe any of them as wacky statistical arguments.
Obviously most of those reports aren't public, but I'm assuming you also have industry access.
Key point: the root cause investigation is not driven by the company involved. The FAA and the NTSB investigate and can by law demand documentation from the company.
The same of course applies to self-driving cars; they are literally cars driven by software, of course you need to do a root cause investigation every time to rule out that it's not a bug in the software that will kill another person (and many after) when the next car happens to go down that rare branch of the system. It's embarrassing to see that the people that call themselves engineers at these companies have not held their work to this standard, and are instead publishing glossy brochures making whacky statistical arguments.