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They don't know what the hell to do with it. In gen 1, it was meant to be a an offensive, beatdown type but in gen 2 onwards they nerfed it down to being more support class feeling, but they've never properly balanced the type and only some of the Pokemon are properly designed around this niche. Most of the time, they lack the versatility to overcome the ridiculously bad type matchup psychic is stuck with and just end up being a bad Pokemon that would be good with a different type.