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.