Michael Chare formulated the question :
I presume cars are made a economically as possible, so is it really the case
that a rear light unit with LEDs and driven by CAN bus, with two thin wires
for the power and two even thinner wires for CAN-H and CAN-L bus is really
cheaper to make than one with convention bulbs and much thicker wires?

The modern system does not make connecting a trailer light socket easy!

Actually, it makes it easier and it can be more clever. One thick wire
as the main supply, then thin wires can be used to pick up the signals.
The trailer lights can be monitored and issues fed as data down the
can-bus.