I would guess it depends on the environment. Energy needs to he expended to develop and secrete toxins, it’s not a free perk.

Plus, you have all the fungi (+ other critters) who cheat and simply look like a poisonous guy to avoid getting eaten. I know there’s lots of snakes and such that do that, there are probably plenty of copycat shrooms.

Did not do biology so don’t know any of the specifics!