US states control registration. Also older vehicles especially kei cars aren't nearly as polluting as one would think especially if only used for work rather than daily use, with mpg in the upper 50 to low 70s though fairly crap nox by comparison to modern 2020s cars.

They also encourage lower speeds which could be a plus in every way.

It takes longer to get places at lower speeds.

That scales with distance. If you're driving 30kph slower for 2 hours, it makes a difference. If you're heading 10-15km to the shop or the creamery, it's not so terrible. Speaking as someone who was a speed freak as a new driver, and sets my cruise at 78 in an 80 zone today...speed ain't everything. You wouldn't want to drive one of these kei vehicles on long trips, anyway.

America's standards for road worthy are just that much higher. IF you: start a car company, buy a bunch of them so the NHTSA can crash test them (thanks Mercedes-Benz for that one), fix them up so they meet modern Federal Motor Vehicle Safety Standards (FMVSS) (which includes backup cameras, among other things), fix them up to meet smog standards (for California), which means it'll also need a compliant OBDII port (and a Japanese OBDII port returning Japanese characters instead of ascii codes won't count), then yeah, it can be considered road-worthy and as a bonus, you can sell them.

It's just that all of the above is very expensive and a lot of work, if not impossible. Having to buy a bunch of cars just to get them crash tested is probably the most expensive part, but the rest of that isn't easy either. Coming back to the kei trucks discussed in the article, there's basically no way they're going to pass crash testing.

So it's not that it isn't a valid solution, just America has much higher standards for "road worthiness".

It's been "done" by MotoRex for R34 GTRs, though there is a bit of fuzziness as to their legality as they got shut down by the government. That's a whole other story though.

But if you're not going to use the vehicle on the road and just as a show car, or drag race car, it doesn't need to be considered road worthy and the import journey is much easier.

I spent some time reading through the US guidance for road worthyness, there seems to be give and take on both sides.

Its probably just that the government has been bought by the appropriate lobby and this has not happened in Australia yet.