I love your question! Probably the civilizations get covered with dirt when rivers flood or landslides fall down on towns. I've never heard about dirt coming from outer space in amounts large enough to cover the land with a thick layer. Volcanoes bury some cities, like Pompeii in Rome. There is probably rubble under San Francisco from the big earthquake in 1906 that destroyed so much of the city. Fires would destroy cities, too, and new buildings would be built over some of the burned old buildings. Also, there must be lots of old cities that just disappeared, because of wind and rain and snow and rotting wood, and because nothing covered them up to preserve them. Not much is left from most of the Old Italian cities that were around at the time of Pompeii, so we don't know as much about daily life in them.

Here's another answer: Some ancient constructions are found by flying over them and seeing unnatural patterns on the surface of the earth. Then scientists dig to find out what is under the surface of the earth that might have been left by people.

Here's a 3rd answer: A cliff washed away near a beach where I lived, and there was an old garbage dump that was exposed when the dirt washed away. That's another way to dig and find out about life in earlier times. I found a strange big round iron plate with a hole in it in the garbage dump. I don't know what it was used for.

Keep asking questions! You can search for answers on the internet at google.com, or you can look in Wikipedia.com on the internet - or in Simple English Wikipedia (simple.wikipedia.org). But sometimes you'll find wrong answers on the internet, too, so keep your mind open to new ideas and new answers.

Sediment blows or washes in from elsewhere that buries ruins, fossils, etc. Elsewhere, rocks and sediment are being eroded away to make this sediment. This is a natural consequence of the fact that rain falls and runs downhill, washing sediment downstream with it. This means that there are parts of the world (such as mountains and many deserts) where rock and dirt are being removed instead of added, but any ruins located there would be destroyed by the very forces that cause erosion as well, so you want to look for ruins where sediment is being deposited instead of eroded away.