Fungi in general are about twice as old as sharks. Roughly a billion years vs ~450 million years.
The point is there just weren’t any which had bacteria to decompose trees, as no bacteria had evolved the ability yet. Until there were. Took millions of years though.
Fun fact, now we have mushrooms which can deal with plastic.
Pestalotiopsis microspora is a type of endophytic fungus discovered in the Amazon rainforest in 2011 which contains bacteria that can biodegrade and break down synthetic plastic polymers.