But a book like that is something that you sink into, the author draws you into their fictional world, over a period of hours.
That's quite different that 45 seconds of outrage the on to the next click-bait, isn't it?
The attention span required seems quite different between the two. I assume that's why people are concerned that things like social networks may be detrimental to focused thinking.
EDIT: I agree that a book like "It", full of suspense, would not ordinarily be considered relaxing to read.