Genuine question - why do headlights need to be on the main CAN bus? Could they not be operated from somewhere closer to the ECU by wires that just carry power and maybe some very simple data lines?

A lot of headlights steer with the car, the can bus is a giant network without any authentication.

It cuts way down on the amount and expense of wiring, copper is expensive. It makes it easier to pocket some more profit.

just wait until wireless CAN comes around

I can see fiberoptics, but wireless just wouldn't be reliable. There are too many sources of noise in a car, plus rush hour traffic with dozens (hundreds?) of cars nearby broadcasting on similar frequencies.

I suspect fancier headlights can pivot based on incline and steering wheel angle.

It may be something as stupid as a wiring harness layout optimization. You can put the headlights controller at the headlights and only run a single set of CAN bus wires (which are probably in that area anyways), haven't played with CAN I assume they are fairly few and fairly thing gauge, through the firewall of the car.

CAN is a 2 wire serial bus using differential signaling. So a couple small wires and maybe some shielding.

It's not about the cost of wire, it's about the cost of software. This is the auto industry version of MCAS. I wish I was joking.

I'm very surprised to see that the ignition control is on the slow speed can bus...

What part of starting a car is fast compared to the canbus? Speed is relative

There are usually two can buses a car, a slow one and a fast one. The slow one controls the lights, infotaiment, climate, etc, while the fast one controls the ECU, driver assist, all related to driving.

If you have access to one you generally have access to both.