In the 1980s, my dad machined a lot of replacement parts for a gunsmith, right here in the UK. All legal, all perfectly legit. I will say it took a hell of a lot more skill than just "download file from thingiverse, press print" - but there's nothing stopping you doing it.

And no-one is (yet) suggesting banning lathes, hacksaws, or files.

The difference is this takes years and years of skill learning and hard graft. Downloading a gun file and pressing print requires nothing.