Asked by: Nikkola Furfaro, Australia
There is a survivorship bias at work here: buildings and monuments left exposed on the surface don’t last very long. Humans steal the best bits to reuse in other buildings, and erosion wears everything else to dust. So the only ancient ruins we find are the ones that were buried.
But they got buried in the first place because the ground level of ancient cities tended to steadily rise. Settlements constantly imported food and building materials for the population, but getting rid of waste and rubbish was a much lower priority. New houses were built on top of the ruins of old ones because hauling away rubble was labour intensive and it was much easier to simply spread it out and build straight on top.
Rivers periodically flooded and added a layer of silt, while in dry regions the wind was constantly blowing in sand and dust. (The Sphinx was buried up to its head in sand until archaeologists re-excavated it in 1817.)
When ancient towns were abandoned entirely, plant seeds quickly took root and created more bulk from the CO2 they pulled from the air. Their roots stabilised the soil created from rotting plant matter and the layers gradually built up.