At least in the US, cars are very much treated as though they are dangerous enough to effectively be weapons. They must be titled, and are required to have an individual serial number (the VIN). Additionally, in order to operate one the operator must be certified by the state to be competent (driver's license). They are recognized as being dangerous enough to require special insurance, and many states require annual inspect to certify they are safe to operate. And speaking as someone who has registered a home-built car in Wyoming (which is both figuratively and literally the wild west for vehicle regulations) it isn't as easy as just rocking up to the DMV and paying for a set of plates. Before registering I was required to have the car inspected by law enforcement for both a VIN and presence of safety equipment. This is the bare minimum, some states require additional inspection for quality of construction and engineering.