Tolkien hides much of the complexity. For example, what happened to the Entwives? What happened that created the Brown Lands, and where did the Entwives go?

Tolkien hides much of the complexity. For example, what happened to the Entwives? What happened that created the Brown Lands, and where did the Entwives go? That could be a whole book, but it's only referred to in a comment by Treebeard, the meat of which is less than a paragraph. That kind of thing happens all over the place. Who were the Ringwraiths before they became wraiths? (Yeah, they were human kings, but who were they?) How did things go down in the war that destroyed Eregion? How did Arwen's mother escape from being held captive by the orcs? What's the detailed backstory on hobbits? And on and on and on. There's all this stuff that is briefly referred to, but never fleshed out in detail. This is useful for authors, by the way. You can be creating a fractal, and you have to cut off detail somewhere. But if you leave hints that there's more detail, the world feels bigger. It doesn't feel amputated or limited.