This is why you see AR15 "80% Complete Lowers" being sold. You buy what is basically a chunk of metal, finish the machining (can be done with a special jig and a drill press), and you can have a functional AR15 that nobody but you has any knowledge of.
The "upper receiver" and barrel, trigger parts kit, bolt assembly, and other bits are just pieces parts and pipe, as far as UPS and anyone else is concerned. When I built my AR15 (out of a locally purchased fully complete lower), I bought the upper receiver assembly online and UPS delivered it to my front door.
Plus, not all states require registration of "normal" firearms. In Texas, the only "registration" is the ATF Form 4473 that is filled out at time of purchase, and those are held by the seller, not the government.
You can build a bicycle much more easily than a drone and many counties and some countries require people to register their bicycles.
There are lots of hunting, firearms, and vehicular regulations that get enforced by cops (and other peace officers) who see things while on patrol.
HAM operators are proud of the strict licencing requirements and the test they need.