How on Earth adding computer and a little PCB of demultiplexor logic instead of multivibrator-controlled relay might be considered as less of conceptual complexity?

I do even doubt in length of wires point. You need a full bus plus a thick wire from power source per every lamp instead of just a one thick wire from relay.

Simple PCBs are extremely cheap to manufacture at scale.

Copper wiring is expensive and heavy.

It’s far more efficient to have a simple PCB controlling multiple local functions (headlights, high beams, blinkers, additional sensors) and a single power/ground pair.

Automotive systems are 12V, which results in high currents. High currents require thick wires, especially in automotive environments with high under hood temperatures where you might have to de-rate wires. It absolutely makes sense to reduce high current automotive wiring.

I don't understand how a PCB+MCU can reduce the copper wiring. The bulbs will consume the same amount of power requiring the same thickness of copper wiring, no matter if it's 10 separate thin wires, one for each bulb, or just one wire, but 10 times thicker (by section area and weight/meter, not diameter).

Common power wire will still require one or two extra wires for CAN, so it would make sense only as replacement for bundles of 3 or more wires going to the same place.

you have a single bus in a ring topology instead of a star network of wires coming from a central location. much less wire and with most indicators and even some headlights being LEDs the current carrying capacity of the +12V wire can be much smaller. GND is the metal substructure and the CAN (or LIN) bus is just two small gauge wires.

much cheaper and much less wiring needed if the bulbs (or bulb holders) can receive commands themselves.

Let's think about a headlight assembly.

Without a board: you need a big power wire for low beams, a big power wire for high beams, a smaller power wire for turn signal. And that's all you can do.

With a board: you need a big power wire for everything. And a two tiny wires for CAN--so you're already ahead. If your beams can move, or be directed, or have LEDs that can be modulated, or have a washer, you start coming out WAY ahead.

Do any cars use higher voltage for power distribution to reduce currents and thus reduce the diameter of wire needed? I'm thinking something like having a higher voltage power distribution network that distributes power to nodes that use a DC to DC converter to provide 12 V to the lights, sensors, etc near those nodes.

24v and 36v are common in trucks and industrial vehicles respectively for exactly this reason, among others. It's really expensive to increase voltage though because all the different components' power supplies have to be designed for transients and supply voltages anywhere from 2-5x nominal in normal operation. Companies will often design up to around 200v, for example.

High power systems do exist, particularly in electric vehicles. They have different challenges to do with being incredibly dangerous to work on.

Tesla have been pushing for a standardized 48v supply system for some time for exactly the reason that 12v 15-30A requires much thicker wiring than a 48v 5A system.

Can bus is a bus. You don’t need a dedicated run of wire per device. You can have a single loop that goes around the whole car that everything is connected to. Things that are “on the way” to others are relatively “free”. Compare this to an independent point to point wire for everything that’s under control.

This is trivially observed if you take a moment to compare a modern day wiring harness to something older, while considering the functionality provided by the later.