It is a format problem, even with reflowable books. Have you ever tried to convert a document containing lots of tables, figures and images to a reflowable document? It's a disaster. We have tested several xml-based publishing systems, but even then, formatting a book is more difficult than many people think.
On a side note, it's quite hard to print books, who has an adequate book-printing machine at home? Someone has to do it.Obviously paper-back publishers do it.