Good thread, lot's of theories from a wide pool of intelligence...
I have a
theory as to why coins only sink so far.
The first thing to consider is that detectors only go so deep and people are willing to dig only so deep, so if 2000 year old coins or otherwise are out of range of detectors it does not mean they are not there.
Also, the activity on the first foot or so of soil will be greater than deeper down. As soil density increases at depth and activity decreases, the coin(object) will begin to slow it's descent. However, activity on the surface could either place more material on top of the coin/object or activity such as fast moving water might wash away material to give the appearance of bringing the coin/object closer to the surface.
I think most coins sink only so far because of much of the activity mentioned by many in this post.
There are many factors which must be considered for differing climates and earth composition in a given area...Wind, rain(or lack of), freezing, flooding, vegetation, human activity.
Scientists(some) have concluded that once upon a time the earth had ONE massive continent...Deserts used to be oceans and mountains lay sleeping deep beneath the crust...
Just another post on an awesome topic for your consideration...
Dig it up before I do!