Because it fundamentally is just a laptop battery. The job of the 12V bootstrap battery is to bring the main control hardware (not even the MCU, I don't think) online so that the DC/DC converter can power the 12V bus from the main battery. There are a few other systems that expect to be powered in the event of a main battery failure, like door locks and charging.
But I really don't think hydraulics like power steering/braking are on that list. The fallback for hydraulic failure is manual pressure, just like it is on any car. It's a naturally redundant system.
But I really don't think hydraulics like power steering/braking are on that list. The fallback for hydraulic failure is manual pressure, just like it is on any car. It's a naturally redundant system.