One of the reasons is that the books are often used in printed format in classrooms and it's important that the page numbers of the ebook match what the teacher

One of the reasons is that the books are often used in printed format in classrooms and it's important that the page numbers of the ebook match what the teacher assigns from the printed copy. With epub format the page # will vary depending on font size. There's probably other reasons as well, but that's one that I've heard mentioned.