Which is where percentages come in.
Besides, you could tax the whole Duchy at the highest rate, and use the extra money to give presents to the Lords that you do like 
Or buy their grain for a higher price. Or pay for their infrastructure.
True - this is what we can do now. However, this takes extra time and effort on the liege's part and requires them to head to a bank to redistribute the money every tax day. I would rather just have it taken care of while I do more interesting things (write fun messages, scheme, train, wage war).
