Most (all?) hybrids and BEVs have a 12V (or 16V) battery. Why?

[VectorBear]

For standby (car powered down), why not have a low quiescent power traction voltage to 12V converter to power standby peripherals such as RFID key fob detector, security system, door locks, computer when the car initially powers up, etc. Once the car does power up, immediately the traction contactor closes and turns on the larger (100-200W?) converter to not only power all of the running 12V system but charge that 12V battery.
Usual arguments are silly: no isolation of traction voltages (converter can be before contactor, with its own protection), drain on traction pack (50-100kWh can handle a few W for a very long time), safety if traction pack fails (but what if 12V system fails in a collision? in the same boat), status quo AKA legacy (time for a new status quo).
So why not replace the 12V battery with a low power, low quiescent power traction voltage to 12V converter to run standby systems?

 

[3DDave]

It seems like they do; they use a converter to power the 12V system when the car is in use and periodically when the car is parked to recharge the 12V battery.

The main thing that separation is for is in the event of a crash, so that the 400-800V is isolated from the rest of the car while the 12V can still operate things like lights and emergency flashers, maybe a phone to contact emergency services all without the chance of any rescuers finding an always-connected electrical system has failed and energized parts of the car to lethal voltages.