smchang
>“If it was too salty, they would be flowing year round,” Dr. Stillman said. “We might be in that Goldilocks zone.”
For instance, if you can't wrap your head around why a language may be missing 'basic' elements, you would never really master that language.
On the other hand, I would guess that when most people read novels recreationally, they are reading in a linear fashion. There is a much smaller need for readers to constantly be flipping to different sections of the book while reading a novel.
There is also the issue of where people read. For textbooks, it is pretty easy to set a large book down on a table or desk at home, in a dorm room, or in a library. It is much harder to do so if you read primarily on a train or bus during rush hour or at a bus stop.