Here are a few reasons:
(1) The state should not have the right to interfere in my personal decision to take (or not take) recreational drugs.
(2) Even if we accept that the state has justification to interfere, prohibition has been proven (with alcohol and now with drugs) to be a dismal failure.
(3) If the subject mushrooms are the ones that were used for certain Native American ceremonies, this is an example of the state interfering with religion.
Psychedelics in general are far too powerful a drug to be approved for use by the government. Alcohol and marijuana are more accepted because they are depressants and slow down your mind. Psychedelics can completely change your perception of reality. While they are not terrible for your own body, a person under the influence of psychedelics is dangerous to yourself and other people.
Watch this video and tell me if psychedelics are safe (not NSFW, bit of language) https://www.youtube.com/watch?v=x2_KEJB4j6A