Because they’re so *easy* to poison yourself with. A lot of toxic mushrooms taste, smell, feel, and look very similar to edible ones. And the “makes you barf for six hours and then it’s over” poisonous mushrooms and the “melts your liver and kills you in six hours” mushrooms also look similar to each other. And some of the less poisonous mushrooms can be made edible if you cook them right, but cooking them right is really hard and you might [trip balls](http://thetinctorial.blogspot.com/2007/05/great-mushroom-article.html?m=1) for a while even if you don’t get sick.
As for why mushrooms as a particular group have such nasty poisons? Fungi are old. Evolution is an arms race between poisonous organisms developing new poisons and other organisms developing new defenses, and fungi have had a LOOOOOOOOOOOONG ASS TIME to develop new and interesting poisons.
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