This is the explanation I believe. It would be easy to say "today's movies aren't as interesting as the ones of 20 years ago" but that would be ignoring that I

This is the explanation I believe. It would be easy to say "today's movies aren't as interesting as the ones of 20 years ago" but that would be ignoring that I changed in those years - namely, I've seen a lot more movies and read a lot more books and watched a lot more TV. It would be unsurprising for someone to say "not everything I liked at first still holds up today." But I apply the same reasoning against my forward-looking watching/reading: "I've seen so much more now that fewer things impress me at first, as I have so much more to compare with." So a book that's only mildly capturing my attention? I can speed up my reading, skim a bit more over the uninteresting parts. A movie? So much harder!