The quickest answer is simply: because that’s how I have received them. The triune God has sovereignly chosen to preserve the scriptures as a collection of books. It seems quite obvious then that he intended they be read as whole books which means they should be read and expounded publicly as whole books. As a preacher, I am a man under authority. It is my responsibility to pass on the deposit of truth which I have received. I don’t have the right or authority to mess with the content or the context. And the context of any passage of scripture is the book in which it is found. So, when I set my preaching schedule, the major determining factor is the fact that God inspired books. So, it would seem, that’s how he intended they be taught. If I were to pick a text from one place and then a text from another place seemingly at random, then I would be sending the message that scripture is under my authority and that I can put it together as I please. And that God may not have gotten it right when he put it together in books. Instead, I want to send the message that I am under the authority of scripture, and I pass it along as I have received it, which is in whole books.
Does this mean I never preach a series that does not proceed through a whole book? No. I am planning to preach a series on why we need the coming Savior during the upcoming Advent season. Such a series, however, should not be the backbone of my preaching. The backbone is the extended preaching of whole books. Feasting on the Word of God as it has been prepared is the meal that nourishes the church.