We have no place to store it. Our drawers are designed to hold only a certain amount of bills and change, so we can't just trade out $20's for pennies because there's no room for it. Secondly, even though it's technical value is the same, I can't give someone their change in nickles and dimes, especially when I'm dealing with a large volume of customers who need to be assisted quickly, or customers dealing with large sums of money. When I need to give someone $10 change, I'm not going to count out 1000 pennies, 200 nickels or whatever.
Secondly, this is a service that you pay for, either through one of those coin-boxes or a bank. I do not make any money off trading in your coins, it's not my job, you want to get your $100 in coins traded into bills? Go to a bank.