Title 17 of the United States Code states that "in the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication." (Or 120 years after its creation, whichever comes first). After that, it's offered up to the people, who do the diligent work of turning it into various horror games.
The pool of public domain options grows with each New Year's Day, when a fresh batch of copyrights evaporates. There are some caveats, and copyright law is distinct from trademark law; you can read more about that and see a list of notable works entering the public domain this year on the Duke Law website.
