Good question.
Well, new soil is constantly being laid down over the old do to volcanic eruptions, earth movement tectonic activities, etc.
While this doesn't amount to much over a year's time frame -over a period of thousands of years it adds up - even over a few hundreds of years. Look at ancient cities, Roman ruins - they are all below curren ground levels.
Coal deposits were laid down millions of years ago.
coal fired placemarker
260 million years or so for bituminous coal. I think the anthracite is much older based on hardness and how folded the seams are today.