"And they do, whenever they perform those skills for an audience. "
The overwhelming majority of musical work -writing, recording, practicing etc - is done in private with no audience present. Many musicians perform very little in public.
That doesn't mean the market values such effort. They have no right to demand to be paid just because something takes skill. Lots of skilled activities are of no value to the market. And I say this as a musician who's very aware of the effort involved.
Not necessarily. They can of course charge before demonstrating said skill, but if someone else happens to be witness, or copy the output in some way, it doesn't mean he controls all future uses of the result of said skill.
The overwhelming majority of musical work -writing, recording, practicing etc - is done in private with no audience present. Many musicians perform very little in public.