I also find re-reading books frequently have diminishing returns, but after some time, you and your world changes, which results in you having a different point of view when you re-read the book.

As you change, the meaning of the book to you changes as well, and gives you new perspectives along with new ideas. E.g. a specific villain or a side character in the book might not be attractive or simply confusing to you, but as you re-read the book, you realize that you get them and they now make perfect sense.