Books (largely) aren't written for memorisation these days, with possible exceptions of some poetry, or children's books.

Classic oral literary traditions are based on rhyme, meter, repetition, simile, metaphor, and references to cultural touchstones and themes, for the most part, all of which strongly assist with memorisation. These are not only necessary for a literature which isn't written down and instead is passed on through generations orally, but also quite likely represents a survivorship bias in that works which didn't exhibit these patterns didn't survive. And of course the versions we know are the end of a long pipeline of transmission (or the world's largest and oldest game of telephone), captured in writing and then passed on to us as (among) the first written traditions.

I've committed a few poems and homilies to memory, one example for nearly thirty years now. I do refresh that recollection from time to time (and realised I was dropping a stanza consistently). It's an interesting counterexample.

Then again, I turned up a piece earlier today I'd written myself about eight years ago and had all but no recollection of.