They often want to grab just the information they need and get on towards a goal. Having to read a lot is an obstacle to completing the goal. Some people think that skimming and scanning is unique to the Web. But it's not. Workplace documents are different from novels.
People use documents.
Even with paper documents, people want to jump in to just the information that they need, grab that information, and jump back out of the document. That difference may be disappearing with high resolution monitors, although we do not yet have definitive research with a relevant task, such as reading and understanding. Many people still find reading from the screen for extended periods to be difficult and tiring. If they are looking for a specific answer, they don't want to see a whole book on line. A single printed book often covers many disparate topics because in print it is easier to publish and distribute one large document than many small ones.