Perhaps it would require too much metadata for each book to add the table of contents so I can choose to select whether I read the whole book from cover to cover or what I consider the actual content, from chapter 1 to the end. If I set a page goal and mark as read a book with not only an introduction, but also the first chapter of the next book as a preview, I'm adding more pages and it feels like cheating.
For example, kindle books already start at chapter one and they will prompt me to add a review as soon as I finish the proper novel, even if there are some pages left, so there is a metadata that says when that part of actual text begins and ends.