Your guess is spot on. It makes it easy to reference. If you look at Wikipedia, with articles and such it cites them by ISBN and page number. Since the Bible is so widely translated into not only different languages, but different versions within languages, as well as printed with varying font sizes, etc, page numbers for such an important book simply wouldn’t happen. Surprisingly, numbering was only introduced as late as 1555. As for the books, they are generally separated into what they were found as. So Matthew is the Gospel written by St Matthew the Evangelist, John is the Gospel written by St John the Evangelist, 1 Corinthians is St Paul the Apostle’s first known letter to the church in Corinth, etc.