To be sure, most people most of the time treat "things as they are now" as the Laws of Medes and Persian, that altereth not. Oral history is very inaccurate after a century or so, even with trained professional transmitting information. So some fantasy/SF characters who act as if their country has had little history are accurate.
But not many.
Because History is often a matter of Vast Importance, but features only such elements as the Dark Lord and the MacGuffin. You're lucky if there were a few changes of dynasty in there. Changes in forms of government? Loss or gain of land? Alliances? Inventions? Invasions? Population shift? Fashion?
Pern is, I think, the most egregious example, partly because "there have been a lot of changes" was made a large issue. But what were the changes? All I remember was that the guilds had become more predominant. There hadn't even been more guilds. There were still the exact number of keeps as there had been four centuries earlier. There had been one military attack that united five keeps under one ruler -- how shocking! -- and it fell apart after his death. I can't imagine a four-century gap with that few changes in the real world. Population pressure, if nothing else -- a heavily agricultural society, like Pern, can expect population growth because children are useful as soon as you can put them to shouting to scare off birds. They would have had to expand to new lands. And why was there only one man resorting to force in all those centuries?