Well, the earth is constantly acquiring new material from outer space all the time, in the form of meteorite dust, but this is a very tiny effect and not the cause of why interesting archaeological stuff is usually found under ground.
The real reason is that natural and artificial processes are constantly moving material around on the surface of the earth. Some places gain material, while others lose it. Wind blows dirt around, and water washes it from one place to another. Glaciers can scrape entire continents clean and deposit what they move elsewhere. Mountains are pushed up by tectonic processes and wear away naturally. Plant material grows and decays, piling up a layer of organic debris (some of which eventually ends up as oil or coal).
You might find interesting archaeological finds right at the surface if they have recently been exposed and no one has looked there recently. Some glaciers have melted, revealing humans buried in them thousands of years ago, with little or no digging needed. There is a bit of "selection bias" going on here. If there were an interesting archaeological object sitting on the surface, someone probably would have picked it up already, leaving only those things buried underground.
Human activity also buries old relics of the past. Have a look at pictures of the Pantheon in Rome. It lies on a level about 10 feet below the surrounding street level. I am told that as Rome has gone through periods of decay and renewal, buildings were destroyed and their rubble simply left in place. Layers and layers of rubble accumulated around the Pantheon, which was durable enough and valuable enough not to let fall apart, so that the end result is that the surrounding city has elevated itself above its old level.
(published on 10/22/2007)