A new software release has driven zero miles. You do not know what kind of performance it has -- regressions both happen and are expected with every launch.
The number of miles a release ends up driving is mostly in simulation + some human-supervised on-road testing, and may be low / influenced by the availability of resources ($$$). There are little to no regulations or 3rd party checks on the process used to clear releases.
They do not simulate every release on all X miles driven. They can't, it's super expensive, and that's to speak nothing of challenges like simulation realism or compatibility of the simulator with multiple generations of hardware that isn't backwards compatible.
They could have no deaths for years, then suddenly a whole lot after a bad software push. Recently they issued a recall because of a bug where the car could hit a telephone pole. Imagine if the software had a bug when their cars were ubiquitous on highways, slamming cars into concrete medians? Imagine if a fault could bork critical technologies like the lidar at a high rate across the fleet?
What happens to Waymo if they kill even 1 child due to a bug or lapse in model quality? I imagine many would wake up to the reality of their safety story and demand a way to audit software releases. Or would you continue to let your kid ride Waymo? This is also to say nothing of sensor failures and other ways the car can mess up.
The probability of your dying in a Waymo is not # incidents / # miles, because past performance does not guarantee future performance when it comes to new releases. It's unknown quantity L (likelihood of encountering a dangerous bug per unit time) * T (time spent in car). Without more data about L (which changes based on many factors, like the company's process for certifying releases), L*T could go from 0 to something unacceptably high really fast.
You trust the car you drive today because the parts are quality controlled and regulated. Software drivers are a very new part added to this chain of things to trust. Their quality control process is not yet publicly regulated. The reality is there needs to be an advancement in technology to ensure software quality / safety, ideally from the role of an unbiased auditor that can catch faults and ground the fleet asap, then restore public faith by ensuring gaps in the qualification processes are fixed. No such mechanism exists today.