This is my own complaint about the fantasy genre, and at least in adulthood, the main reason I can't take most fantasy books. For all practical purposes every zombie novel and movie is the same, and to a large extent most fantasy novels are too. Why? Because the former are about crowds of hungry stupid people with strong constitutions chasing protagonists in various cities. (I've seen this in the real world, in Amsterdam; it's called "When the coffee shops close".) Most fantasy novels seem to be transparently the early Middle Ages, with the names of the countries changed, and the reports of miracles and magic taken literally (but the Christian language and symbolism taken out), complete with barbarians on the borders that the authors must realize look a little too much like Saracens sometimes. Often we even have the looming shadow of a fallen empire. The genre's inventor was a medievalist, so big surprise there! In Maresca's post, reader Daniel jumps in to slice and dice the setting's level of technology and figure out the closest real-historical parallel, but this would seem to be a short-lived amusement. Maybe if the writer is making a point about the development of politics or technology this would be more worthwhile, but usually it seems to be a mix-and-match costume put on top of the story-line. Imagine Celts with biotechnology! Imagine curiously Charlemagne-like knights with laserguns! Why!
Above: the latest genre: 1971 Indiana fiction. Indiana-in-1971-ist K.S.S. Klutzenmeyer turned his lifelong interest in the history of his home state into fiction. Alert readers may notice similarities to 1971 Indiana, except the people (called "Khoosiers") have green tiger-striped skin and ride pterodactyls, and are super mystically awesome! Below: the cover art for the first, second and third novels in the series.
As to why medieval European times specifically are more interesting - have there been Inca scholars that tried to do the same and their stories didn't spawn a genre? - I don't know, although to be exciting to us moderns I think you need a) some kind nation-state identity so you can engage the tribal loyalty circuit, b) 1:1 combat, c) recognizable values that you think were carried by people who you think basically looked like you (even if they weren't) and d) it's far enough away and long enough ago that the endemic disease and violence in these times and places can be ignored (hence, no fantasy in modern sub-Saharan Africa, because it's hard to escape the realization that it's just miserable.) All of these suggest thought experiments in genre-bending. Sure, we have pseudo-Sumerian work (does Conan count?) and even paleolithic fantasy, but not whole genres. Do we have "honest" Christian fantasy? C.S. Lewis, like most fantasy writers, changes the names. Why not write about mythical Christian kingdoms, using the power of Christ to blow away enemies? (This may seem distasteful to modern Christians, although the cynical among us might say that's a way to avoid thinking about why we can't use Jesus magic, if it ever was used this way.) Or how about a fantasy/Picaresque novel by an escaped slave in 2012 Mauretania? Somewhere around the third robbery and gang-rape readers' attention might start to wander. Or flee.
The Tower of Druaga is supposedly a Sumerian anime fantasy. I mean come on, it looks like they didn't even try.
In the end, it's a damn shame that when someone is writing speculative fiction where they're allowed to bring the setting into play and use it for any authorial purpose they want, and they restrict themselves to early medieval, period. It's as if you win the lottery and instead of building your dreamhouse, you furnish your house with all the prefab Välms and Squerms and Zalfs from Ikea. (Ikea is fine, just not where I'll shop when I win the lottery. And writers have unlimited imagination dollars.)
Worth pointing out: as long as I'm implying a fantasy vs. science fiction opposition it bears mentioning that science fiction stories are also not innocent of this problem; that is to say, writers don't always stop to consider why they're using the setting they're using, or what they're going to do with that setting. My favorite example is Asimov's inversion of racial class status in The Stars Like Dust to explore his country's own race challenges; a lot of social conventions become much more obviously ridiculous when you're presented with a counterfactual. And it's harder to do this in fantasy, because the restriction is greater - when a genre is defined as using one historical period as its setting, I think it's going to get in trouble eventually. (Hence why I love China Mieville's fantasy work.)
All that said, yes there are things that are innately cool about swords, dragons, rayguns and aliens, and sometimes things can just be cool without being allegorical - but we all have our limits as to how long the formulas can remain interesting. One adolescent I talked to was particularly excited about fantasy novels, and made clear, "I don't like unoriginal fantasy novels where it's the same old dragons. I like fantasy novels where dragons come here, to our world!" Clearly this young man has a very low threshold for depth-of-innovation and doesn't mind reading practically the same book many times, although I imagine it will rise as he gets older.
Speaking of Mieville, have you read The Scar? And have you seen the real seasteading proposal that's moving forward? We may have a real floating polity soon, although hopefully it will expand only through voluntary trade and not piracy. If you're aware of Mieville's politics and remember the execution of the officers when the new ship was taken, this real-world instantiation is not without irony. (Mieville's linguistic science fiction book Embassytown reviewed here.)