How does all this stuff get buried this deep in dirt?
Dirt is not a fixed commodity. The world is always making more of ith Plants get into cracks, they live and extend the cracks eventually crumbling walls. When they die they compost. storms toss debris on top of it all and you get more compost, more broken stone, etc etc. Over centuries stuff gets buried.
In the case of Ancient Rome, the forum was built in a low swampy area between the hills and the Romans drained it to build the famed forum, the heart of the empire. When Rome fell, the drains went into disrepair, clogged, and the forum returned to swampy mess. Eventually the debris was such that it formed a hard surface that people could walk and build on and only the very tops of the 20 or 30 foot Roman columns stuck through.
Nature doesn't care about our architectural triumphs. Time and entropy are all it needs to tear them all down and bury them 100 feet deep.
The Incas had a Deep State, too..........................