Because 3D printed guns are extremely effective at skirting current firearms regulations. There are designs out there that are mostly 3D-printed but use a few metal components that are easily made otherwise or modified, like a pipe that’s reprocessed into a usable barrel. They can be made completely untraceably by anyone with a few commonly available tools. Hence the name “ghost guns”. If your area has something like an assault weapons ban or a license requirement, ghost guns make them irrelevant.

The only way to effectively regulate them would be to target 3D printers themselves, and that’s far from a perfect solution. Making everyone with a 3D printer become a licensed gunsmith would be insane. Just as bad would be mandatory content scans for file sharing sites similar to what’s available for CSAM. New York has resorted to just arresting anyone they find out has a homemade firearm.