I fail to see the difference between SaaS and book sales. People are still working to sell new copies of the book you wrote 10 years ago - it needs to be printe

I fail to see the difference between SaaS and book sales. People are still working to sell new copies of the book you wrote 10 years ago - it needs to be printed, distributed, sold to the customer (and maybe shipped to him afterwords). In the same way, SaaS needs to be maintained and operated. In both cases someone gets to do the ongoing work, and someone gets paid for the original work. Why do you think they are qualitatively different?