There are several different contributing factors. One is that cities tend to get built on what are called depositional zones. These are places where sediments accumulate (as opposed to places sediment erodes from). People like to build cities near rivers and near the mouths of rivers....just the sort of places where rivers flood and dump a bunch of sediment. This naturally buries things over time.

The second is that, especially historically, rubbish built up in cities. There were no trucks to haul away rubble on a large scale. If a building collapsed, you just sort of knocked down the rubble and built a new house on top. Especially if your houses are made of stone or mud brick. This results in a layer on layer buildup of debris that can actually leave an artificial hill called a tell.

Also, there's a flat-earth levels of crazy conspiracy theory about how the whole world was flooded with mud about 100 years ago, wiping out evidence of some advanced globe spanning civilization and burying the lower levels of cities. It's not the truth, but you may run in to people talking about it so I figured I would mention it.