Because semantics. The location of the "next" page is a property of the entire page, and therefore belongs in the head (similarly, so does the title of the page

Because semantics. The location of the "next" page is a property of the entire page, and therefore belongs in the head (similarly, so does the title of the page). It's not a property that may be present in any of the links (or other elements) of the page. There's a whole lot of advantages that follow, but aren't quite reasons why (e.g. like a physical book, I don't need to read the page itself to figure out where the next page is), but it follows from applying the semantics of webpage (meta) data in a consistent manner. (except for a CYOA book)