SO ...
Egyptian ruins were in the perfect place to be very well conserved. Indeed, they were near the Nil river and they slowly got cover by the regular spate of the river, which brings sediments and others sand particles. In the other case, buildings were in the desert so they simply get covered by sand, thanks to the wind. Moreover, the Prehistory only began like 3000 years before our era : so we're far from the 6000 years Himring stayed desert !
Roman and Greek ruins are, for the oldest one, only around the V or VIth century before our era.
Footprints are a special case : the ground need to be wet/loose, like in a marsh or near a river. To be conserved, they need to be quickly covered by sediments before they disappear. It's impossible to compare Footprints and man made buildings.
And finally, Himring had very bad preservation conditions :
-an earthquake because of the Fall of Thangorodrim, and it should be stronger than any earthquake that we ever had on earth
-and surely a megatsunami due to the earthquake