So, fungal spores are literally everywhere, and the requirements for fungus to thrive seem to be trivially low; give it a moderately humid environment and it’ll grow on a bare concrete wall ffs eating god only knows what; the dust from the air maybe?
Well, and the great outdoors is full of slightly damp places, many of them downright soggy most of the time - and absolutely rife with organic material to snack on.
Where’s the bottleneck? Why isn’t the world a choking fungal hellscape?
I have read somewhere that shiitake, and many other mushrooms from Japan only fruit when shaken hard because it has developed in an earthquake prone country.