With computers, prices dropped and the learning curve wasn't much of an issue anymore. CNC Mills and lathes still require a much greater skill and up-front cost than a 3D printer needs. With CNC machines, there's usually still an additional necessary step required to turn a model into a tool path, based on your machines. So you still have a learning curve no matter which way you cut it (pun intended).
I don't think 3D printing will be a viable way to distributively mass-produce weapons, it's the fact that somebody already has been able to produce functional weapons with 3D printing, and it's comparatively much easier buying a $10K mill and generating tool paths.