Boarstorm said:
Well, heck with it. Let's assume that either:
a) There was something MORE behind the fall of those Empires whose falls we do know about; or
b) The reasons behind ALL the lost civilizations are lost to history.
In addition, let's make the following assumption (regardless of a or b, above):
c) The fall of every single one of these empires is LINKED in some way.
Now then, with these assumptions my mind immediately jumps to a third party who is, in effect, purposely keeping the world in its PoL status for some nefarious reason of their own. Here's where I've hit a brick wall.
Who would want to ensure that the peoples of this setting never develop a real degree of civilization? Who would be capable accomplishing this goal? And, perhaps most importantly -- WHY? Why would they want to?
To an extent, this has happened in the real world. Empires rise and fall for a number of reasons - cultures change, and what made an empire successful at one point disappears. Economic resources dwindle or trade is catastrophically broken, plagues cull the population and weaken the ability to respond to crises as well as the existing social fabric. External invasions happen, and an empire is conquered by force. Sometimes a people just get tired. Sometimes this all happens at once.
However, there's got to be more to it, because, in general, the real world has become more populous and more technologically advanced over time. Certainly some powerful deity could be keeping things in check, but I prefer a more mechanistic explanation. Keeping in mind that the existing forces bringing down empires are already at work, what could be causing these failures in our D&D fantasy world?
* Magic: The magical equivalent of global thermonuclear war. High level rituals that cause massive disasters are possible by the highest level people. Genius and madness often go together and, perhaps more importantly, such rituals may be possible by only one person. All it takes is one powerful psycho and nobody to stop him, or people who think this massive power can be controlled. Good for at least one fallen empire - see Netheril in the Forgotten Realms.
* Multiple Intelligent Races: In the real world there were multiple species of proto-humans, but only one survived. Why? Perhaps humans in particular just don't play well with others. For all the death and carnage in our world, there are strong ties that bind us together on a deep level. What would happen against other races where those ties aren't there, particularly the ones that look non-human? Genocidal wars can certainly take down empires, and this jives with what's known about the Bael Turath empire in W&M.
* Monsters: Simple and easy: monsters of all types breed faster than the common races' ability to kill them. It takes constant vigilance to keep them in check. One provincial administrator's idleness can lead to the decay and overrun of his province, potentially causing huge repurcussions across the empire, weakening it fatally against some other threat. Even worse, when the empires fall, things get REALLY bad, because it's up to the locals to keep themselves safe. If they can't do it, they lose and the place becomes a wasteland.
* The Planes: Perhaps the world is swiss-cheesed with portals like Faerun, has planes that move in and out of alignment like Eberron, or perhaps the walls between the planes are just thin in certain places. Heck, maybe there's a plug somewhere that people keep disturbing. This may make certain types of monsters stronger, cause huge natural disasters in the case of the Elemental Chaos, or just let loose some of the big badasses that nobody in the world has the capability to stop. Why do the empires fall every 2000 years? That's when the Elemental Chaos is coterminous and elementals flood the world along with drastic natural disasters. Sounds bad, huh? Well, every 10000 years the Abyss is coterminous and the seas turn to blood, the locusts come, and the demons reap their grim harvest. That's worse.
* Environment: Maybe the world just isn't as nice a place as Earth is. What we consider to be natural disasters could be relatively common occurances in this world. Hurricanes, earthquakes, ice ages... it's only a matter of time before a civilization is torn down. The survivors pick up the pieces afterwards until the next calamity.
* Disease: Many of the diseases humans suffer from started with animals we've domesticated and then jumped over to us after a mutation. Well, what the heck kind of diseases do you get from griffons, displacer beasts, giants, and grells? Maybe the magical ecology of the world is such that the balance of power between us and diseases is different. Heck the Black Plague killed 1/3 of Europe. What if stuff like that happens a lot more often? What if it's let out on purpose by creatures from other planes? Even if it's not enough to bring an empire down (and it would have to be really bad to do that), it could still weaken it so thoroughly that the other external pressures take it down.
* High Level Destruction: Similar to the first option, but a little bit different. Maybe magical apocolypse isn't too easy, but the cumulative effects of lots of high level people running around and at war with each other can have much the same effect, especially if there's no easy counter. Meteors dropping, earthquakes, ruined crops. Alone these are just horrible occurances, but two nations inflicting these on each other constantly would be catastrophic. Sure, there's Mutually Assured Destruction, but that only works until one side thinks they can win...
Personally, I prefer a combination of all of these. Due to the magical influences on the world, is a DANGEROUS place. Heck, maybe the real question is how the empires start overcome all of this in the first place? Maybe the "homelands", the places that have always been populated and rarely get demolished are places where the walls between the planes are thick. It's easy to hold the line there, but it takes constant work elsewhere. No Reapers needed!