I always assumed it was so directors could get both the book audience (the fraction who will watch the movie no matter what, as long as it has the book name in the title) to watch the movie, as well as the median movie watcher. It is less money-making if a movie trades the bulk of its audience for a tiny sliver of the leftover book readers (who wouldn’t otherwise watch the movie), since in the grand scheme of things, people don’t really read that much.
I always assumed it was so directors could get both the book audience (the fraction who will watch the movie no matter what, as long as it has the book name in the title) to watch the movie, as well as the median movie watcher. It is less money-making if a movie trades the bulk of its audience for a tiny sliver of the leftover book readers (who wouldn’t otherwise watch the movie), since in the grand scheme of things, people don’t really read that much.