Why do I do this? Because it's fun and I like it.
I was telling my wife how much I would love to tell our future kids completely original stories set in a world of my own creation. Sure they'll get the regular fairy tales and Arabian Knights and King Arthur, but they'll also get stuff no one else gets. Whether it's ever published or not I would get a kick out of it if my kids take the ball and run with it, spinning tales of their own in my setting. I think of R.A. Salvatore and his son Geno now partnering to tell Drizzt tales 20 years in the making.
Another thing I love (and hate) is when I do something and then later come across it in the writings of other famous authors, i.e. Tolkien, George R.R. Martin, R.A. Salvatore. I love it because it gives me this little ego boost that I am thinking and creating like the greats. I hate it because it means anyone who has read them and then reads me will think "he totally stole that from..." I will state clearly that the only fantasy I read prior to beginning to build was The Hobbit and LOTR (after the movies came out) and Robert Jordan's Wheel of Time. So any similarities to those are probably subconscious. But that's what I'm getting at - subconscious archetypes!
I checked out a few books to kill the long weekend with and the first one I read is loaded with insightful references to "modern fantasy" being so popular because it mirrors ages old archetypes. Celtic deities and faeries, Norse elves and dwarves, Shakespearean sprites and witches. Carl Jung and Joseph Campbell basically state they're buried in every human brain the world over, that's why LOTR was one of the most read, influential, and beloved tales of the past century. And it's not losing momentum anytime soon because people are eagerly awaiting The Hobbit trilogy (I am so excited it's more than just one movie!) and I'd bet that many of them who will purchase tickets haven't read the books or perhaps weren't even alive when the first trilogy came out over 10 years ago.
The Silmarillion is one I've yet to complete cover to cover because it's Tolkien overload. I can't process more than a chapter or two at a time. I have read The Hobbit a few times. I need to re-read the LOTR books. And The Children of Hurin was one of my favorites.
What I'm saying is, we read [fantasy] to escape the mundane world and get a glimpse into the sacred, mythical worlds that our human experience so seems to lack. Some of us build such worlds to get even further into the experience. And sometimes unintentional, almost unavoidable mimicry is going to take place. But they say imitation is the sincerest form of flattery. We do what we do because we are compelled to. Our inner elf is forever trying to break down the divide from the physical world and the faerie realm.
Some interesting tidbits to finish with: the words yourself, ourselves, themselves come from the long held Celtic belief that humans are descended from elves. That's why Tolkien's tale of Aragorn, Arwen, and their son Eldarion resonates within us. And over 30% of modern Americans have Celtic ancestry from Europe's days gone by. The Celts also believed that if someone makes you think "hm, that person looks like an elf, faerie, dwarf, etc" it's because they have that race somewhere in their bloodline. It's the same way that you can very closely resemble an ancestor for 5-6 generations ago.