Imagine reading Moby Dick, and the author goes into a long boring diatribe about 19th century cetology. If you're sitting at home reading a physical book, pushing through is a long, difficult process that requires continuous effortful willpower. But when you're running a 10km, at worst you just zone out util the chapter's done. It's not like you have anything else to do. Sure, it's not optimal in terms of reading retention, but it's better than giving up completely because you can't force yourself to turn the pages.