Try googling this, from ars technica. It's a gentle read:

"Why was most of the Earth’s coal made all at once?
It wasn’t because fungi couldn’t break down early trees, study argues."

And then there's peat, which is a bit like coal (all right, so it isn't made from huge great Carboniferous trees...) that hasn't been buried, squashed and warmed up a bit. Plenty of that here in Cornwall...Wales...northern England...Scotland...Ireland...everywhere coolish and damp, really...Those fungi would have it all if they could, but they can't because of environmental barriers..low pH...no oxygen... Geddit?