You have that backwards. You can't patent a game or game materials (these can be copyrighted or trademarked or both), but you could patent game mechanics b/c they were "processes" which were patentable under the Federal Circuit's non-existent standard for patentability. In fact, several game mechanics have been patented.

Whether those patents remain valid is another question entirely. In 2007, the Supreme Court struck down the ridiculously low bar for patenting business methods and software processes, which includes most game-related patents.