Why are ancient buildings, cities and structures usually found buried underground?
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Imagine you drop a coin in your backyard. It sits on the top of the soil and if you come back tomorrow, unless someone has moved it or it’s been very gusty out, it’s likely going to be in approximately the same place and visible above ground. Maybe it’s a little dirtier than when you left it. After a little while, the wind will pick up enough dirt (etc.) to cover your coin, grass may grow around and over it, disappearing it from view. Progressive layers of everyday natural and manmade materials will build up over time and make it seem like your coin is getting deeper underground (when really the surface level is rising). Still, the coin is only going to get so deep underground while you’re still living on the property and maintaining your house and garden.
If you move out and centuries pass, however, and nobody maintains your property, it will eventually start to crumble and new layers of dirt, rubble, and overgrowth will cover your coin. There is now a little hill where your house used to be. Future humans, passing through, see a nice hill by the river you used to live by and think, what a lovely place to live, and conveniently close to the water! They build their homes there for a while, drop more things of their own, and then move on in time and the cycle starts again. The hill gets a little bigger.
Many cities in very strategic locations have many many layers of habitation. These can form visible mounds if people keep building over each other’s remains. The oldest material is usually the lowest, the newest material (the coin you just dropped) is at the top. This can be disturbed however by things like earthquakes and agriculture (e.g. while plowing, you realize fragments of ancient pottery are being brought up to the surface as the earth shifts – you’ve found an archaeological site below!) You may also find newer material below if someone above has dug a large pit for dumping rubbish or dug deep burials, for e.g. But in general, think of a layer cake: the top layer was laid most recently.
One more thing: different environments will result in speedier / different patterns of coverage. Drop a coin on the beach and come back even a couple of hours later and you probably won’t find it without a metal detector!
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