I forget where I heard or read this but for something like millions of years wood and other plant materials was not broken down by anything at all as there were no animals to eat away at them.
And it went on for about 60 Million years which seems an unfathomable timespan.
https://en.wikipedia.org/wiki/Carboniferous#Rocks_and_coal
So it wasn't mountains of wood that lay on the ground during the Carboniferous period, it was like, the best mulching job ever. The trees pwned all other species at that time. Can you imagine being a weed trying to grow up through all that bark? You'd have one hell of an up-hill battle!