Because the medium is actually the same. The content of a book is not paper, or a cover. It's text, and specifically the information in that text.
LLMs are intended to directly compete with and outright replace that usecase. I don't need a textbook on, say, Anatomy, because ChatGPT can structure and tell me about Anatomy, and in fact with say the exact same content slightly re-arranged.
This doesn't really hold for fictional books, nor does it hold for movies.
Watching a movie and reading a book are inherently different experiences, which cannot replace one another. Reading a textbook and asking ChatGPT about topic X is, for all intents and purposes, the same experience. Especially since, remember, most textbooks are online today.
And, I think, elephant in the room with these discussions: we cannot just compare ChatGPT to a human. That's not a foregone conclusions and, IMO, no, you can't just do that. You have to justify it.
Humans are special. Why? Because we are Human. Humans have different and additional rights which machines, and programs, do not have. If we want to extend our rights to machines, we can do that... but not for free. Oh no, you must justify that, and it's quiet hard. Especially when said machines appear to work against Humans.