I first read Lord of the Rings when I was a teen. I loved the story, but wasn't interested in any of the deeper lore and philosophy. I just wasn't ready for it.
Re-reading the books as an adult, I am struck with how much they have to say about the nature of change. See Galadriel didn't have to leave middle earth at the end of the third age because of any rule forbidding her from staying. No she left because her ring no longer had power, and without that power she could no longer keep her world from changing.
Elves are immortal; elven rulers rule forever; elven gardens grow forever; so long as there are elves to tend to a location, that location never needs to change. Yet, middle earth is not static, it is a place of change. Kingdoms rise and fall, peoples live and die, forests appear and then disappear as the landscape itself changes.
Some elves can live within this maelstrom of change for a while, but eventually they all grow weary of it and travel back to the undying lands by way of boat or bodily death. We the reader are never invited to this place for the same reason other mortals are not invited; they are boring.
Every time the fellowship finds themselves in an elven realm, be it lothlorien or rivendell, it is only to rest before moving on. The plot can't continue in these places because no plot can exist in a place where every day is the same as every other. Humans subjected to that would eventually fade away in the same way that elves fade when subjected to endless change.