Virtually all electronics need a step down (buck) converters as they run at lower voltages 5, 3.3, 1.8. 12V > 3.3- 1V would a single step. 48V ones would likely require an intermediate step. The only exception would be running some power systems where it'd require less current.
You're going to need the expensive bits of a power supply anyway to meet transient requirements, so it's not much of a savings to run at native voltage and it gives a lot of design freedom/reusability to have one voltage for everything.
The main savings is current though, because the wiring harness is one of the most expensive parts of a car.
The move to 48v is very much about efficiency within the harness backbone. For the same wattage, less amperage is needed in a higher voltage system, meaning the wires can be smaller and they produce less waste heat.
There are a few different topologies for a 48v harness, but somewhere in the line there's a 12V DC/DC converter in there somewhere.
The wiring for 48V can be a lot thinner than it is for 12V. As there is a square law involved for resistive heating it turns out that wiring for 48V can use 1/16th of the weight of copper as that for 12V.
A switchmode converter can be designed for 48V just as easily as 12V.
It's far cheaper and easier to just pluck a readily available 12V power supply off the shelf than it is to design one that will have limited applications outside of a single manufacturer.
Virtually all electronics need a step down (buck) converters as they run at lower voltages 5, 3.3, 1.8. 12V > 3.3- 1V would a single step. 48V ones would likely require an intermediate step. The only exception would be running some power systems where it'd require less current.