Same reason that it’s uncommon for any page to have most of the page covered in ink, regardless of whether it’s a book or a sheaf of papers or whatever. Ink costs something, and it’s cheaper to put ink on a little bit of the page than it is to put ink on everything but a little bit of the page. Unless there’s a compelling reason to do otherwise, you take the cheaper route.
Same reason that it’s uncommon for any page to have most of the page covered in ink, regardless of whether it’s a book or a sheaf of papers or whatever. Ink costs something, and it’s cheaper to put ink on a little bit of the page than it is to put ink on everything but a little bit of the page. Unless there’s a compelling reason to do otherwise, you take the cheaper route.