The power of a puzzle
Why do QAnon conspiracy theories (and Dan Brown books) fool people despite being easily disproven? Because on the internet, if you only search for what you want to be true, you’ll always find “evidence” validating your beliefs:
The reader no longer needed to rely on the experts to determine whether the book was a gimmick (and maybe couldn’t trust the experts either, if the conspiracies are correct!). The reader could go to Google and find articles of undetermined quality and unverified accuracy in order to form their own opinion.
The ultimate genius of “The Da Vinci Code” wasn’t in its bad writing or its poor plotting; it was in the book’s ability to allow the reader to LARP being an investigator and religious scholar to uncover arcane knowledge that “they” don’t want you to know.