Probably would still need the 12V battery to have enough charge to close the connection to the high voltage battery that would power the step down converter.
But yeah it seems dumb to me that most EVs don’t keep the 12V battery topped up from the high voltage battery somehow while the car is parked, but I’m not an electrical engineer ¯\_(ツ)_/¯
I think it’s mainly there just to be able to control the circuit that cuts power to the high voltage battery off while the car is parked for safety reasons.