A CNC mill that's worth the cast iron it's made from weighs at least 2000 lbs, not to mention it takes a lot of skill to use (workholding, toolholding, setting up feeds and speeds, coolant, etc). It's very easy and very expensive to crash if you don't know what you're doing. A g-code program has to be modified to fit your machine, where the origin is, the dimensions of your rough stock, what tools it expects to have, how much material your machine can hog off.
In contrast, a pretty good 3d printer costs $500, can sit on a table, and the inevitable mistakes you will make while learning how to use it are comparatively cheap.
You can buy jigs to complete what are called 80% receivers with a drill press (and (optionally a router) - could do it on your kitchen table in an evening for a couple hundred bucks.
Gun frames can be made out of plastic or aluminum, and there are fixtures for benchtop CNC machines that can be used to make them. This is not nearly as complicated as you make it sound. I think Cody Wilson was basically selling a turnkey solution for that, maybe still is.
AFAIK they claim to still be selling general purpose CNC machines that aren't marketed as being for firearms... but only take the money and ghost customers without actually delivering anything.
In contrast, a pretty good 3d printer costs $500, can sit on a table, and the inevitable mistakes you will make while learning how to use it are comparatively cheap.