They are. All someone has to do is include in it a statement to the effect of 'I waive my copyright interest in this work and hereby donate it to the public domain.' There is no bar on voluntary placement of work into the public domain; it just doesn't happen that often because when publishers write checks to authors they're purchasing the right to exploit the work commercially. Authors could grant publishers limited licenses to do so and then stipulate that the work would fall into the public domain after a certain date, but that would result in lowered payments from publishers, plus it creates an incentive for the publisher to under-market the work or even withhold it from sale, and then publish it once it enters the public domain without paying anything in royalties.
There are some publishers that make good money of just republishing historical works, even without exclusivity. Some people want a good-quality facsimile of an old book. Dover paperbacks springs to mind.
The reason others are owed rights is that copyright, as with patents, is a contract between the state and the creator/inventor. Part of the deal is that creators get copyright protection in return for the eventual entry of the work in to the public domain - they pay off the demos with that promise.
State doesn't owe creators of works protection naturally. If creators want to not have a limited right to exclusive use they should be allowed to give that up and then the state should exclude itself from any actions against people who copy that work.
To recapitulate: if you don't want to 'buy' copyright protection with eventual movement of your work in to the public domain then you can instead have that work freely copyable - without tort - based on the ability of anyone who sees/uses it. Without ready movement of works in to the public domain the public are being ripped off, the contract is broken and the public then should consider those works copyable without the possibility of a claim of tort.