Notice how a book smells like old paper and maybe even coffee spills. That smell alone can pull a memory or a whole scene into your head before you even read a word. Tapping a clean screen doesn't have that same dirty, human feel to it. Your brain makes weird links from those physical bits that a perfect app just can't feed it.
My friend found a flaky old bus ticket in his dad's beat up notebook. The faded date and destination spun into a whole story about a missed trip. His app's blank page never would have given him that.