Although I can't personally imagine re-reading YA lit, it still requires you to use huge amounts of grammar, vocabulary, and cultural knowledge in a seamless automatic way. To imagine an alternate world, people, and culture, while your eyes scan constantly across page after page invisible to your captured mind.

Those are the same language tools you need to use to create and evaluate your own writing. You're using a mental tool at a high level, even if it's not Ulysses (or Homer's Odyssey). This is fundamentally different from passively viewing cat videos, or thirst traps, which may well not involve language at all.

A 2 year old can be entertained by short form video, but can not even sound out a single word of a book.