For plumbing, building codes dictate the range of solutions that are available for any given problem, and they're not secret (but buying a guide can be expensive.) However, painters, plumbers, and other trades definitely have tips and techniques that they've learned through experience and from mentors, and you won't find much of that being given away freely. You've got to learn it the way they did, through hard work and apprenticeship. And that's ok, because those skills require mastery to use properly, and that's true in software development too.

Exactly this. My father is a civil engineer who's now in his early 60s, and he recently got to work with colleagues much younger than him.

He told me that said colleagues didn't know some trade-related stuff which hadn't got covered in university courses (like how to reinforce earth ditches when digging in mountainous terrain with probable heavy rains) but which was paramount in keeping the work going (not much that you can do once a ditch collapses because of heavy rain). He told me that in this instance he had instructed his younger colleagues on what was the correct procedure to take, but he also told me that he's got other professional secrets that he's not ready to share just yet.

I'm curious enough to ask: why?

Do you see musicians making samples and giving them away to other musicians for free? Sure, you can find some online but it is definitely not the norm.

Does Hollywood create stock footage and release it for other production companies to use for free?

Do photographers allow others to use their photos for free?

OSS is very unique to the software world.

Magicians!

(an exception that proves your rule of course)

The difference is that construction has long been a commodity industry and software has not become one yet.