Handling cash ain't free. At a minimum, a certain amount must be kept on hand to make change. Then theres insurance on that cash. Then theres the safe (and registers) you have to buy to store the cash. Then theres the transport of the cash to the bank.
Also, better hope your cashiers never make a mistake or accept a forged bill.
Also, better hope your cashiers never make a mistake or accept a forged bill.