Making actual firearms from steel is trivial. In fact you can look up Kalashnikov designs online and then replicate it with a relatively simple mill/lathe/tapping setup.
To be honest, as someone familiar mostly with computers, I have no idea where to start with this and it sounds a bit intimidating. Buying a 3D printer and using some 3rd party design sound very easy in comparison.
You're certainly right that it's possible for someone determined to make their own firearm, but raising the bar still has immense value.
Tradeoffs between freedom and safety are another, unrelated discussion.