Presumably, though, those are also things that you'd need for a physical print book, and you could share some of that effort / cost between the two? Maybe this is a naive assumption, but I would assume that the total cost per unit of ebook is still a decent amount less than a print book, even when factoring for the publishing costs, purely due to the fact that you are dealing with a digital version compared with a print version that needs real materials and labor.
But they do need editing, layout, design, marketing, etc, which are a significant part of the cost of a book. A book is never a solo endeavor by an author.