Well, here in the real word, there are tons of ancient Egyptian ruins, Mayan ruins, castles from the Middle Ages, and old forts and fortresses. Are we in a post apocalyptic era?
Well, kinda. I would say we are living in a post-post-apocalyptic era. We are not living in the immediate aftermath of a particular apocalypse, but there have been many . . . the collapse of classic Maya civilization, the fall of classical Rome and the ensuing dark ages, Tamerlain, WWII, just to name a few.
Like for us, it seems that the landscape of Cyrodil is littered with the refuse of history.
As for the abundance of hostile bandits, and conjurers, and trolls, etc., that is another story.