There are many reasons why the order has been using ghostwriters for their books. The most important one is that the most influential books cannot be published under a real name of the order. This is why the entire production of the book is basically given to a human ghostwriter. All they need to do is use certain phrases and words from time to time in their stories. This is enough to get the subliminal messages to work on the minds of the people who read these books. The actual sci-fi contents of the books should be highly engaging and attractive in order to capture the attention of the reader and make sure that they read all of it.
For the subliminal messages to stick for a long time, the entire book length is required. This happens because the messages are strategically scattered through the book. The length of the subliminal message material will determine the length of the book that needs to be written.