For a paper book. But for an electronic book it's the opposite. An author/publisher has no idea about the size of the screen of the device the book is going to

For a paper book. But for an electronic book it's the opposite. An author/publisher has no idea about the size of the screen of the device the book is going to be read on + many people like to tweak the font and its size. And you don't need the actual numbers as long as you have a clickable ToC. Page numbers can indeed be very useful for the purpose of non-electronic referencing of pieces of a text, especially if a printed edition of the book exists you need to collaborate with people having such but I would rather use simple anchor tags for this.