needing to keep a step-down (plummet down, at those voltages!) voltage converter continuously running off of main batteries to power some background electronics would be pretty inefficient.

the batteries wouldn't like it. where-as alkaline & lead acid batteries are efficient at low-discharge, lithium ion batteries generally don't enjoy low-discharge . buck converters are also probably going to be more efficient when they have some modest load to deliver- otherwise the drive circuitry & mosfets are consuming some (little) juice, just to produce a trickle of power.

in general this is an automotive vehicle using automotive rated electronic parts. having a conventional 12v battery-powered architecture for most systems makes loads of sense, is is hella convenient, and based off my above reasoning i'd guess more efficient. yes with that low-drain the inefficiency feels like it probably won't matter but I rest easier thinking that the li-ions can truly rest when the car is off. it's better for them.