It is a cost issue. Paper and the distribution of it is relatively expensive and the margins in the newspapers are razor thin (hilarious i know) on a product that is cat. Printers are just trying to sell as many millimeters of ad space as possible. Especially since nowadays the largest papers in many countries are the free Metro style papers.
For books the production and distribution costs are obviously a lot lower relative to the retail price and they don't make money selling ads in books. You can easily spend a couple of bucks on a nice quality paper, good margins, binding and a hard cover.