Break it down to amateur parts changer's, then gunsmith's that are going to run machinery to work on the firearms to customize/repair them.
So not only may you be working a gun that costs in the 10's of thousands of dollars to replace if you screw it up, but working a machinery that you can do that much damage to the machine in less than a minute as well.
Bluntly, most of your time starting off in a technical school will be learning first how to run the machinery without crashing it or destroying the work piece to being with, then how to use these skill to gunsmith afterwards.
Let the first page scroll through the photos on top, and you get a feel for the classroom/where you are going to spend most of your time just to learn how to use these tool correctly (so you don't crash them, or kill yourself instead).
http://www.schooloftrades.edu/
And to prove this point, lets start out with a easy one, being a revolver that has too much BC gap.
Barrel and cylinder need to be removed from frame.
Cylinder may need to be faced to remove pitting, so we begin by facing the cylinder.
Next is the barrel and its shoulder need to be faces so it can be threaded into the frame at least one wind, it not two winds more (depending on the amount that the cylinder had to be faced) and correctly index to the frame when the barrel is reinstalled to the frame. Now with cylinder back together and back in the frame for measurements, the breach face of the barrel needs to be faced to achieve a .006" BC gap when installed correctly, and the forcing cone re-tapered as well.
Granted that there is much more to a revolver rebuild/BC gap correction but, as you see, this is a high machining job, with little to no parts being replaced with new parts, hence no amateur drop in parts changing.