Because you don't really have to do any additional steps once the ink is on the paper like you do with a 3D printed firearm.

Looking at just the silhouette or shape, you couldn't really tell the difference between the body of a firearm that was later going to receive a metal tube from the hardware store or a plastic bracket that I'm going to use to support a shelf.

And even if you could accurately tell the difference, how do you know I'm not helping my nephew build a Nerf gun?