/lit/ - Literature

r/books is basically the terminal stage of the commodification of literature.
As a commodity, literature sells the idea of being intelligent. The commodity is closely tied to physically reading the words with your eyes, because Audiobooks are a relatively new innovation and haven't had time to integrate into culture.
If these people cared about the contents of the book rather than the action of reading in in of itself, they wouldn't give a shit about what other people think of their preferred reading format. But, again, r/books consists of people who consume literature as a commodity, not as art. If people aren't validating their actions, then there is no point to performing them. And these consumers are self-conscious, they know they don't read to learn but to "be a reader". Just look at all the wacky relatable memes about le epic book stacks.
By telling them that audiobooks aren't physical books, you are devaluing the commodified personality they have adopted, and treat it as a direct attack on their character, because they know no difference between the products they buy and their inner self.