every neighborhood has a few people who can build you a car from scratch in a day

How many cars does the city demand be produced each day? Cause my own city of Houston sees over 380k new cars sold annually.

the city already has several thousand car factories that don’t require registration

Itty bitty ones, sure. And they take weeks to produce a working vehicle, of substandard quality.

there’s no mechanism to enforce registration

Until you get pulled over

the cost of building a new car is a few hundred dollars

A plastic car that can go maybe a few hundred miles before falling apart if it's made perfectly.

Building a 3d printer from a kit is different from building one from scratch

You can build cars from a kit, too. But I'm not going to do that when I can buy a used Corolla or KIA for couch cushion money and drive it into the ground over the next fifteen years.