The skill is in being able to solder pipes together, something that is useful to learn as a general skill. It doesn't even need specialized equipment, just a torch, some flux, and a spool of plumbing solder. There is also a tiny bit of electrical work, but that's literally just matching the colors and using a couple of wire nuts.
The building code stuff is more of a bugbear.
> The skill is in being able to ...
Admitting that one of my grandfathers was a blacksmith and the other a farmer, that my own father could have built a house from the ground down (I helped him dig and pour a foundation once) and then up, etc. - most modern Americans just don't have the backgrounds/aptitudes/comfort levels needed to try and succeed at such tasks.