Why does fungus on expired goods make us sick, but not the other edible mushrooms?
Because they make different poop, in the case of mold.
Edible mold make waste products that won’t hurt us, but dangerous mold make things that are toxic when they eat things.
The mold on blue cheese has been selected over hundreds of years to be both harmless and tasty, and it pushes out other molds when it grows on the cheese we leave for it to snack on. Any random mold spore you could pluck out of the air might be harmless, but might be dangerous.
For fungus it’s similar, but as larger organisms they can make and store toxic compounds on purpose and not just as waste.
Control, control, control. There are mushrooms that you can eat just fine. There are mushrooms that will kill you. There are mushrooms that will make you see things that aren’t there. There’s thousands of species of mushrooms and if you get the wrong one it can be capital B Bad.
So mushroom growers are very careful about the kind of mushroom the grow. The fungi that grows on your bread? Honestly, we have no idea what it could be and don’t want you to find out the hard way it was the Bad kind.