I think this is mostly true for older worlds (or at least 100+). We all know that the seed that comes from birds, is usually left on the ground, eventually truning to rot. Now i think that would be a great if that rot would turn into mushroom (or at least disappear bcus its kind of annoying on the ground).
Thats all, maybe it could be true for beefalo poop too, that would make sense cus irl. there are mushrooms that grow from cow manure.