That's a good point, but my copyright-1980 dictionary uses pages to describe what it is and how to use it, and knowledge of how to use a dictionary was much more common back then than it is now. Plus, dictionaries had been around for centuries.
Let's add: some other books in the 1980s had what-is-role-playing sections. That didn't stop bored house parents from thinking that the books were cult recruitment tools. Had (when?) the issue gone in front of a judge, I expect the what-is-role-playing section would be an important part in establishing an author's case that she's not out to turn the nation's reading youth into Satan-worshipping, spasmic-dancing, midriff-baring, empty-air-conversing . . . oh no! It's happening!