Dark nve against Fight, se against Psy.
Fight nve against Psy, se against Dark.
Psy doesn't even hit Dark.
Psychic gets the worst deal here so it would be regarded as unfair; It has to be fair for all types.
Without incorporating Fire/Water/Grass, fairness can't be achieved. Also, those three are the 'elements' that everyone used to believe matter was made from (excluding air) so it would be stupid to deviate from those types.
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