Then I'm surprised. It's much different to the usual situation when even mass made products are either banned or very limited, e.g. unicycles prohibited, e-scooters only up to 20 km/h, e-bikes only up to 25 km/h.
I agree, it's surprising. Even in California of all places, you can make a one-off vehicle at home and register it for legal road use, without even needing modern safety or emissions equipment:
Another even more common strategy is to "restore" a classic car using some extremely small number of parts from some really old pre-emissions and pre-safety equipment car. This is often done for hod rods, dune buggies, etc. where it will be, say a "1930 Ford" but contain only some minuscule amount of that original car it is titled and registered as. There's a sizable industry of homemade "kit cars" that require you to start with a legally registered VW Beetle, but ultimately they often retain nothing except parts of the thin sheet metal floor pan, and somehow that is apparently legal.
Regarding 3D made parts recently there was an accident: https://news.ycombinator.com/item?id=46152941. One may expect increasing regulation. At least in the air.