You know how a sidewalk, driveway, etc will get grass and trees in the cracks that you have to clip? And how the edge of the grass can creep over the edge of the concrete, meaning the sidewalk occasionally has to be edged to keep it clear? Same thing, but with a lot more time.

Worth adding that cultures would often knock down old buildings and just level the remnants, then put the replacement building right on top of the older one just a foot or two higher than the predecessor. Many of these are the big mounds you see that are listed as archeological sites.

Combine these two for generation upon generation, and you end up with ruins buried anywhere from a few centimeters up to several meters under what we now call ground-level.

Of course, wind and water can move dirt in -- and they can remove or erode it as well.