I've even been to a bunch of ruins and ancient dwellings in the US and I had to look up their names.
and to be fair all the interesting stuff is in the natural history museums, which people visit _a lot_ in the US.
Indeed, old is not enough. Old and grandiose (at least for that epoch) is what attracts curiosity.
For example, something like Stonehenge could be assembled in a matter of minutes with modern machinery, but several centuries ago it was more than the state of the art. Furthermore, it hints to a knowledge of astronomy which is daily knowledge in the epoch of Hubble, but is astonishing considerind that the first telescope would have to wait for Galileo to be born.
Another example: in Italy there are plenty of majestic cathedrals. We may discuss about the utility of spending so much money and man-centuries on cathedrals, but they are just so beautiful that you get speechless once inside.
It's a bit isolated though, unless you really want to see Four Corners (which is completely uninteresting) it's a serious détour from the Yellowstone — Salt Lake — Canyons axis from which you'd likely turn west rather than east (or turn east south of Grand Canyon to Petrified Forest then go back west immediately). And sadly I don't remember anything of interest in the Navajo reservation (back when we went through anyway) which stands right between Grand Canyon and Mesa Verde.
I'd visit that, has anyone done it? Would be great to have some of the burial mounds created so you could walk through and see them from the inside too.
You could have a "moving of the stones" festival where people could come and camp and have the opportunity to take part in moving and placing some megaliths. That'd be fun! (Kinda like a Society for Creative Anachronism (SCA) project.)
Obviously, Peru's ancient/older indigenous monuments are very impressive and common. I used to live within a 20 minute radius of a handful of them.
Second, Peru being a former Spanish colony means a good amount of impressive cathedrals, town squares, and old palaces/wealthy people homes.
When I moved to the US I moved to Texas and people told me to visit the Spanish/Mexican historic buildings, including the Alamo... they were the same kind of building that were so common in Peru they were turned into banks.