I think it's the same problem as with most genre's these days to be honest. Writers get caught in a 'formula', based on the older 'classics' because those classics, when they came out, were fresh, exciting, and new. Which is why, say, Lord of the Rings is considered an incredible example of the genre, but so many, many books that follow are in the 'It's okay or pretty good, but has it's problem' to 'oh god, please not ravening orc hoardes vs. the Happy Shiny Elves again!'. I honestly think many fantasy writers borrow the bits and pieces of legwork done by successful writers before them, give them what they think are new and unique twists, and call it good.
Tolkien's world, for instance again, took decades to build.
Many fantasy writers? Months, maybe a few years, but it stops as soon as the underlying structure is 'good enough' with hints and suggestions, abd that's it. And I honestly think that's a big part of the problem. Not enough time spent in the world-building stage...and not enough time and willingess spent later during the writing of the story to adjust, add, take away from the initial frame work to help make the world richer.
I'd compare it to, say, having a stage backdrop as opposed to a full, cinematic background. The backdrop is fine if you've in a small stage production, have limited room, and are depending on your costiming--I would actually think the parallel would be a short story or mere chapter (or even part of a chapter) of a book.
You want a whole novel though, or, to bring it back to the background analogy, a full cinemtic event, you're going to have to get out there and get three dimensional backgrounds, show things like trees blowing in the wind, crowds in the background that have nothing to do with what's going on with your character if you don't want the place to feel like a ghost town, weather that changes, climate that changes as you move, lighting for different times of the day, etc and so forth. Evevn the same area has to change in details as time goes on if you don't want to give the audience the feel of being back in a theater watching a play set up against a fixed backdrop.
Unfortunately, I think most authors are using the backdrop instead of the full cinematic treatment.
...Man, I hope that made sense. I haven't had enough coffee yet. :)