If I do a work for hire whether it is building a deck or writing an app, I get paid and that's it.
If you do a work for hire, there's one person who's paying you. If you are an author, there isn't one person who just pays for everything -- you actually need to sell the book again and again to make back the time you spent on it.
If I create a new product, I get patent protection for 20 years and that's it.
There's a huge difference between copyright and patent.
Patent law was designed to prevent factory machinery designs being kept secret. Factory owners were inventing better and better machines, but keeping them secret so that they would retain competitive advantage. Some designs died with their owners. Patents protection was invented to encourage inventors to document their creations while preventing others from using them, but then to allow the next inventor to create a better machine without being blocked by the patent.
A book isn't a machine or technique, so it's not like technological advancement is hanging on being able to use a copyright work.
(That's not to say I don't think copyright terms aren't too long.)