> And yet the obvious thing is for someone to be making and selling a "can bulb" - a tiny 4 pin bulb with 12V, GND, CAN-H/L pins.
No, that's not obvious at all.
Separating the control board and the bulb is obvious. You wouldn't want to replace your entire control circuit every time you need to replace a bulb, would you? You don't want to have to reprogram your ECU to know which bulb serial number corresponds to your front headlight because all of your bulbs are the same.
Moreover, this is impossible because there isn't a single bulb model that goes into a car. High beams, low beams, blinkers, and interior lights are all different. They also differ from model to model depending on the requirements.
> That then makes it vehicle specific, so you don't get the economies of scale of just making a single model of can-bulb which fits lots of places in many cars from many manufacturers.
Car companies make millions or tens of millions of cars per year.
When you're making 10s of millions of something every year (or 2X that for parts that come in pairs, like headlights), you already have economies of scale.
Automotive equipment manufacturers will also share components between car companies, and further upstream you have companies that make chips for auto makers who share chips across the companies.
Automotive manufacturing is a great example of economies of scale. It's not correct to say that auto manufacturers aren't leveraging economies of scale while producing 10s of millions of common parts per year.