One other explanation is that many of these monuments didn't stand around undisturbed for 2,000 years, a lot of work of hundreds of generations of humans went i

One other explanation is that many of these monuments didn't stand around undisturbed for 2,000 years, a lot of work of hundreds of generations of humans went into these monuments. The Porta Nigra is a great example - it's an ancient Roman city gate in Trier, Germany, still standing since it was built in 170 AD. The ground floor was buried for a long time (>800 years?) preserving it. The gate was a ruin for a few hundred years, around 1000 AD a church was built around the gate keeping the construction in place. As part of a general de-'churchification' 1802 Napoleon had the church-part taken off and had the gate and its insides fixed up (while the ground floor was still buried!). So the gate didn't have to survive on its own.