While this is technically possible, it is not that easy. In other words, someone who is technical and experienced enough to manually create a lower like that is very likely to have extensive experience with firearms anyway (and likely owns many).
You'd be surprised how motivated someone in a gang could get watching a ton of videos on youtube just to get access to a gun police cannot "trace" in a meaningful way.
> While this is technically possible, it is not that easy.
Isn't the same thing true for 3D printers? The first time someone tries to print something they frequently end up with spaghetti and less technically competent people wouldn't even be able to get the thing to attempt printing anything.