Why do we not find coal deposits in rock greater than about 443 million years old?

Coal forms from the accumulation of plant material in swampy environments, which decomposes and transforms over time. The lack of coal deposits in rocks older than about 443 million years is due to plate tectonics and the movement of continents, which have caused older coal-bearing rocks to be buried too deep, where the heat and pressure have transformed them into other types of rocks.