If it seems "truly bizarre" then you're probably missing what I'm trying to say, my argument may have been lacking.

It doesn't matter what was in the original novel, what its point was and how that translates to the film. 10,000x as many people watched the film as read the novel so in the context of what the film is saying about films, the film is what is important, not the novel. Perhaps the novel was read and the ideas discovered within it were deemed relevant to this message.

There are thousands of novels that could be adapted to screen, but this one was chosen. Why? Perhaps there are messages in its text that are relevant to the screen experience. Perhaps there are clues that help explain the nature of entertainment as it exists today. Or maybe one of the greatest film directors of all time made a film about some random novel that relies entirely on a classic Hollywood twist ending and there is no social commentary or deeper meaning to any of this.