I think it was a pretty common pre-modern (not by any means extinct today) academic view that the arc of history was generally towards the decline of civilizations. Like our ancestors were great and amazing and we're just kindof pale imitations of them. It probably doesn't hurt that when the two most important sources of authority in your society are noble title/blood and religion, your ancestors are much closer connected to the source of those.

Modern fantasy picks up this trope, where the most powerful magic and the greatest structures etc are always in the past, only being rediscovered by people in the now.