>What makes these works so much more valuable that their creators should never have to give up the rights to the public, the way that creators in the USA for nearly 200 years did?

Nothing makes these works more valuable, its just the jurisprudence has caught up to modern applications of the law. Copyrightable work is an asset much like anything else I should be able to leave to my heirs, why should I be limited to making money off my work during my lifetime or any other arbitrary period of time. Especially because there is case after case that establish work that becomes popular only after the death of the creator.

So I will ask you your own question turned around, why as the creator of a work should I ever be forced to give up my rights of the same to the public?