Why won't the fortify alchemy enchantment work on your enchanted gear in Skyrim?

The Fortify Alchemy enchantment in Skyrim does not work on gear that is already enchanted because the game mechanics prevent stacking the same type of effect on an item. Enchanted gear cannot have its own enchantments enhanced by other enchantments, meaning that the Fortify Alchemy effect will not apply to items that already possess enchantments. Additionally, the Fortify Alchemy effect is meant to enhance the alchemy skill and potion potency, rather than directly affecting enchanted items.