Why is it that archaeologists must dig through layers of deposits to uncover previous history?

Alright! Everyone tells me that the m of the earth is constant, with the exception of falling space debris. However, does this account for the multitude of geological layering that archaeologists must uncover to discover the structures and other material evidence of previous civilizations? If not, then what natural process is it that causes the covering up of previous civilizations? In your answer--- Please exclude the obvious causes, such as: volcanic activity (ash, lava, etc.), or landslides, floods, sand-drifts, etc. I'm seeking to understand in those cases where such geologic changes have not occured. THX!