Many western countries, even with longer histories, don't have a national identity that is that much older than the US. There was this 19th century idea of deliberately building a national identity that swept through the world, that in many ways superseded any prior identity that merely happened to exist. So even if buildings and ruins may be old, the identity itself is often surprisingly young. It may hark on events from the 18th or even 17th century, and tack on some fairy tales of brave knights or ferocious vikings, but it was more often than not penned about the same time the US national identity began to crystallize.