Because despite what some people here say gamefreak just tacks on weaknesses and strengths of certain types like poison and dark in whatever way they see fit for balancing or whatever, seems like they made poison pretty weak early on but at least its a little better now.
A Psychic person would know how to avoid poison due to being psychic. You can't poison the ground. You can slowly do damage to earth if you intentionally poisoned it (IE, Nuclear waste) But that wouldn't be very effective and would take too long.
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