Because that would cancel the literal everything. Removing everything from the Drops List is safer and leads typically to less exploits.
In your case I would check if the drops List contains any sort of “oreCopper” and replace that with your quantity of Raw Copper Items. (your current check is for if the harvested Block itself is a Copper Ore Block, and not the thing that got actually dropped)
This will not affect Inventories like Chests and such, since they drop things differently and not on Harvest.