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.

 

[VectorBear]

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.

In a collision, the traction contactor can and should turn off, so no HV outside of the pack. Quite safe, simple and reasonable. However the HV pack still exists intact or you have other major problems such as a mushroom cloud forming over your car. Why can't the traction pack still power those emergency ancillaries, safely stealing power before the contactor? This assumes that the 12V wiring is intact, reasonable assumption, circuit protections pop if shorted. See the attached sketch.
Not previously mentioned, the batteryless 12V buss can still be jumped from another car, but I don't know what use that would be.
Side note, years ago I was a witness to a collision where a 12V car battery passed us on the road after the cars abruptly stopped. Impaired driver hit another car, I was a witness in court, convicted, also the bartender who didn't know when to stop serving.