The map of middle earth isn't in the middle of a philosophical treatise on understanding the nature of the physical world and the composition of the universe..... :/
It's rather in the middle of a story populated with (demi-)gods interfering in the affairs of men, mythical creatures, and heroes of elvish descent. And hobbitses. Filthy, nasty hobbitses!
And Utopia is.. it’s perfectly valid genre. Inventing a fantasy world to convey your theoretical ideas in a more approachable way seems like a very sensible approach.
For that matter Tolkien to claims that his world is actually set on our planet just thousands of years ago.
