On top of all that, almost all the wiring in the car can be made thinner, because of the greatly reduced losses. This saves a bit of weight, but also a lot of cost because copper is expensive.
There is no free lunch. You need to go to a finer wire strand, better insulation, better loom, better support for the harness etc, if you want that super fine wire to last. Some of those have pretty direct labor cost impacts too. That's gonna kill a lot of your cost savings, especially at lower production volumes where the design cost is harder to amortize. There's no free lunch.
It's very, very hard to get insulation that's not good for at least 100V and I suspect that just about any generic wire is good for more like 300V.
The only exception that comes to mind is wire that's specifically for "household low voltage" like 24V AC for thermostat, doorbell, sprinklers, landscape lighting. Also normal ethernet. But these are almost all what you'd call signalling wiring rather than power wiring.
Your average hook-up wire that you could buy at the auto parts store to make some repairs is almost certainly rated for 300V already. Mostly because of chafe resistance. Wikipedia says that the dielectric breakdown strength of PVC is 40 millions volts per meter https://en.wikipedia.org/wiki/Polyvinyl_chloride.
Divide both sides by 1 million and you get 40 volts per micron. OK so you need 1/3 of a micron to insulate enough for 12V and you need 1.25 microns for 48V. Now let's have a reasonable safety factor of say 10 or so and we're looking at 3 microns vs 12.5 microns. The only wire I can think of that might have insulation that thin is enamel coated magnet wire for the inside of motor windings. But even that is probably thicker.
Any kind of plastic insulation is going to be significantly thicker than this just to be able to be coated onto the bare copper wire and stick.
You're not wrong that the insulation needs to be thicker as the voltage goes higher. But you're unaware of just how ridiculously over-insulated everything already is due to other constraints of manufacture.
