This enchanment can only come from fishing that what makes it rare even with the new open water mechanic. This enchantment is able to be enchanted on a shovel which when dirt, grass or paths are broken it has a chance to drop loot e.g. name tag or slimeball. The enchanted book with excavator has a 0.05% chance to drop from fishing.
I believe this should be added because it gives you chance to get items that you can't get in the biome you are in.