I can understand why. I'm guessing it's for a couple reasons, maybe fluctuations in the voltage depending on driving conditions ( if you're stomping on the throttle allowing everything to flow through the motors it may provide inconsistent voltage to the sensitive computers and electronics, I would imagine there is a step-down converter somewhere that charges that 12v battery, essentially that battery is used as a buffer. But the link between the big batt. and little batt. isn't active unless the vehicle is on. And "On" requires the 12v system to turn on computers and close a relay.
Doors relying on ANY electronics is a bad idea. Even most cars with keyless entry have a hidden key somewhere to physically get in the vehicle if the battery dies. If the main battery in a tesla is toast you have bigger problems than a locked door. But anyone who has been driving for more than a few years has likely dealt with an OG battery decides to stop taking a charge. And you probably won't get much of a warning in an EV that doesn't have an engine that starts turning over slower and slower.