Wind, water, and worms - predominantly.
Remember also, that most of plant growth is water, air and sunlight - only the small amount that remains of a tree after you’ve burnt it to ash is from the actual earth...so when plants overgrow something, they add organic material on top, and then collect blown dirt or mud. Then animals poopon top, and track dirt all around. Earthworms in particular keep piling little bits of soil on top of the surface.
Seems sketchy to me. Wouldn’t you think wind and rain would UNcover as much as it covered? And it seems like an awful lot of plant matter would have to accumulate to bury something under feet of soil. And what about areas that are only marginally fertile and don’t support much plant life?
I’m not disagreeing, but it just doesn’t feel like those factors would be enough to bury entire cities in a few thousand years.