Earlier quoted context omitted.
Actually, they very much do: 1. As a relative marker of a current position. 2. As an absolute number when your ebook/reader screws up saving its state.
1. Is solved by "N Pages left in chapter" which is more useful than "Page N" 2. There is no "absolute number" of pages on a e-reader app or device because the window or font size can change, changing all the page numbers. You can have 100 pages or 200 pages.
2. Most users read from fullscreen readers on their phones or ebooks, so the window size doesn't really change. I also strongly advice setting on a font for a particular book and stick to it. From my POV (unbacked by science) it helps with recall. If you don't change the font or other layout settings, the "pages" retain their numeration.