I caught a post recently wondering at how a civilization could possibly last 5000 years, when the most notable civilizations in our world don't last near that. I think I have an intelligent answer: magic is what makes the difference. Non-magical plague hits? Cure disease. Bad farming? Create food/water. King dies tragically leaving power vaccuum and infant son? True Ressurrection. Feel bad your civilization got destroyed and want it brought back? Wish (and hold onto your pants!)
I went back and thought about how civilizations last as long as they do, and suprizingly the question comes--how do they ever end. There's so many ways that you can duck almost any fate that I'm amazed anyone even dies in D&D other than from old age. You'd figure once people caught on that worshiping Pelor or whoever meant free food forever and the ability to bring your dead wife of ten years back from the dead you'd have more clerics than farmers. Okay so the low aptitude ones (wis. 11 types) would only be healers, water makers, food purifiers, etc. but in our world even a single cruddy cleric would singlehandedly raise a community from decline to plenty.
Now imagine places where sorcery and wizardry are taught at academies, where temples churn out clerics to wander the land and see to people's needs. Kyuss could come back and it would still be fine. Yeah sometimes people catch a bad worm and turn into awful critters for a while, but hack down the abomination and raise it again and you have old Whittick back good as new. No reason to fret the Age of Worms. There's still plenty of food and good lighting, and if anything bad happens to anyone it can all be undone.
I think that there is the real problem.
Just speaking from a Forgotten Realms standpoint, the giants, dragons, and elves had long lived societies in part because they are long lived creatures, but that is farther back in history, so I'll move forward.
You are completely correct. A lot of "real" empires fall apart because the original founders die off and there are multiple claims on the throne, and the kingdom then breaks up into smaller, less viable entities. In D&D, liches ruling a kingdom can be the one true emperor for a long time, and epic level characters can extend their lifespans as well (though this isn't as well explored in 3.5 as it was in previous editions).
But as to why empires don't last longer with magic around, look at both Greyhawk and FR and we can see what happens. I'm sure after the Invoked Devestation there were many common folk that were not thrilled with wizards, and even those cultures that came from Netheril in the FR often either abandoned magic or they decided to use much more "toned down" magic after their major problems.
Humans have a tendancy to keep reaching for more, and with magic this tends to lead them to clash with powerful forces that may be a lot more than they can handle.
But thats just my take.
I agree that a longer lifespan means longer kingdoms. Also, it depends on what you mean by "civilizations." If you mean an independent country, then yes, the average lifespan for any notable kingdom is typically a few centuries, far less than a thousand years and certainly never five times that. If you mean a race's ability to build such kingdoms and be a moving force in the world, that's somewhat different. Human civilization on Earth has been around at least 5,000 years, and there are some who believe it goes back much, much further.
As to ways an actual kingdom survives, it does have to do a lot with having magic, but more to do with the genre being fantasy. There's no real way to say, "No, a kingdom can't last that long." All the author of a certain history has to do is explain that this kingdom happens to have overcome all the troubles in its past. Wars? It won them all. Plauges? Priests stopped them. Assassinations? Loyal supporters and strong families came to the rescue when ressurections weren't an option. And of course, the general answer for any such situation that is always applicable- some other adventurers did it!
In our real world, no country has had such good luck as to always come out on top of whatever forces confronted it. But, if the world builder wants, their fantasy kingdom can, and who's to say that they aren't right?
Second Saern's comment on "depends what you mean by civilizations."
Look at China or India, and you can see lots of cultural continuities going back at least 4,000 years, maybe more if the archeologists are right. There are some significant changes in both places, but civilization (in the sense of a set of cultural traditions) doesn't just disappear entirely when the Yellow River floods or the Mongol hordes invade.
Even looking at civilizations that have supposedly been conquered and wiped out, they're often still there, lurking under the surface. Lots of villagers in the mountains around Mexico city still speak the language of the Aztecs. They don't do human sacrifices anymore, but they have mixed a lot of indigenous beliefs in with their Catholicism. Has Meso-American civilization disappeared, or simply been overwritten and blended with new ideas brought by outsiders? Have the conquerors won out, or been absorbed?
Done in by ecological disaster? Sure the Hopi and Zuni of today (or at contact with the Spanish) didn't have as huge and illustrious of an organization as the Anasazi, but there is a cultural legacy from the Anasazi there.
Did civilization end when Rome fell? Well, things in parts of the empire got pretty chaotic, and not many records survive from the centuries after the fall in those areas, but Byzantium preserved a lot of the traditions and passed them back to the Italians during the Renaissance. Not to mention all the Latin-writing monks hiding in their monasteries and preserving classical learning. Yeah, there's a drastic period of cultural change, but we still, with some reason, talk about "Western Civilization" as a continuous phenomenon stretching from the ancient Near East to modern Europe and the U.S.
If you're looking for political continuity, that's a different thing. There are almost always ups and downs that cause ruling families to change, along with the political institutions, the boundaries of states, and so forth. Large empires, especially, are difficult to hold together. But frequently memory of "the golden age" prompts people to try to restore what they see as the way things ought to be, and so states and empires get reconstituted. Even foreign conquerors find themselves forced to adapt to the political institutions of the conquered people, whether in China or Mexico or Ireland.
In a fantasy world, lots of more apocalyptic things can happen, so if anything, civilizations might have a greater chance of being truly wiped out or at least drastically reshaped on a regular basis. On the other hand, elves and dwarves ought to be able to maintain continuous civilizations for tens of thousands of years, as long as they can adapt somehow to the great climate changes that happen on that time scale.
I hear your point Grimm. Honestly, there are very few dissasters large enough that magic can't put them back together again. Indeed, if FEMA had a few mages to their name (even low level ones) huricane Katrina would have been a completely different story. However, as tragic as such things are, they rarely topple nations. Usually people pull together and life goes on. Historicaly the one element that has toppled more nations than any other is the human element. When you put too many people together, the worst parts of human nature begin to crop up. Greed, pride, lust for power etc. No matter how many fancy toys you give to people, they are still people deep down. Maybe their nation will last a little longer, but it is just a mater of time before internal corruption spreads and they fracture and fall appart.