Yeah but at some point people decided to stop registering them en masse, which created an entire underground economy
You believe that point was after RAPBACK?
Oh, sweet Summer child!
Prior to the Gun Control Act of 1968, homemade firearms, or "Privately Manufactured Firearms" (PMFs), were 100% legal with no federal controls placed on them. As of 1968, if you intended to sell or distribute the guns you made, you had to have an FFL and each gun must have a unique serial number.
It wasn't until Aug 2022 that the BATFE enacted a rule to categorize "a weapon parts kit that is designed to or may readily be completed, assembled, restored, or otherwise converted to expel a projectile by the action of an explosive" as a "firearm" -- the same way a receiver or frame is categorized for purposes of registration, background checks, etc.
I view this as the opposite situation than people en masse not registering. Instead, the rules by which the game is played were changed so that previous PMFs were now being treated differently. Private Manufacturers were suddenly transformed from law abiding citizen to firearm felon overnight based on no actions on the part of the individuals.
This is the slippery slope the gun control zealots have been using. Each time they make it more difficult to buy a gun legally, PMF numbers increase. Each person they ban from owning guns -- sometimes for being convicted of a non-violent, blue collar crime like insider trading -- the government adds another name to the list of potential customers shopping in that underground marketplace. With the legitimate sources of guns becoming more restrictive, it's only natural they would turn their focus to the less traditional sources: DIY kits and ready-made ghost guns.