You don’t have to serialize firearms you make* for your own personal use. You also don’t need to register them either*. In fact there’s restrictions on the federal government’s ability to keep a registry of guns. 18 USC 926(a)(3). From a policy perspective it’d just be creating another possession based crime that’s almost impossible to enforce. Because you could just pop a “1” on the side and claim that you sent in your registration paperwork but the government screwed up.

*True for title 1 firearms (most handguns, shotguns, and rifles) not certain other classes like those that machineguns or silencers fall into.

https://www.atf.gov/firearms/qa/how-does-person-register-firearm-or-remove-name-firearms-registration