Wither Enchantment

Why don't Wither Skeletons drop swords that deal wither damage? This proposal will add a new enchantment that can be found in loot chests in nether fortress, or on the stone swords dropped by Wither Skeletons. This enchantment can only be put on stone tools and inflicts wither on attack. This enchantment will inflict wither based on the level of the enchantment. Note that this enchantment is very rare, so it's not as OP.