3D printed guns are NOT illegal to manufacture in the US. Any normal gun(non NFA) can be manufactured for personal use at any times. This means no SBRs or full auto weapons, essentially. Most guns do not fall into this category and are totally legal to make. This is firstly due to the fact that the US has the 2nd amendment. secondly, disallowing such manufacturing would make metal pipes illegal because a pipe shotgun can be accidentally or intentionally created this way which would essentially outlaw benign objects like that. They only need to be serialized when they are distributed(sold, gifted, etc) because they requires an actual manufacturers’ license.

Of course, the world does not revolve around the US, so for many parts of the world, yes it could Be illegal. However, those that are manufacturing these are either doing so explicitly knowing it is criminal to commit crimes or also to rebel against tyrannical governments like in Iran. The second case is where the guns are used as intended and the first case encompasses a broader issue regarding why someone would want to commit a crime with them to begin with.