Although some things are undoubtedly buried on purpose, many other items, large and small, found in an arcaeological dig have been buried naturally. Some in dramatic fashion, such as by vocanic ash, earthquakes, wind driven sand and dirt, slumping or sloughing of nearby higher terrain, etc. But, just the natural day by day accumuation of dust, dirt, grass and weeds will, over the eons, cover up the signs of every day life. Think about what your house looks like if you fail to dust for a few weeks. Additionally, abandoned buildings eventually collapse in on themselves and become flattened making it all the more simple to be covered...