What your post described is calling growing up. It's not an absence of imagination or desire to enjoy content. It is a choice an adult makes to not be consumed by an arbitrary object or emotion. Being able to compartmentalize and move on is a good thing, not a bad thing.

Adults are fully capable of enjoying a story, and also living their actual lives without needing to have the story they're reading consume them. Children "live through the book" because they're learning about themselves and their values, and humans learn by imitation.

When you're an adult, you have your own style of living and you don't need to do it through the "lense" of a book. That doesn't mean these books don't influence an adult. Several books I've read in adulthood stick with me daily and I continually think about them. But they are works of fiction and they don't dictate how I live or perceive my life.