Just like people who use UDP because it's more efficient, and then have to re-add all the stuff TCP has. It looks like Bitcoin has to re-add all the protections that the old money system has.
A cash register has things incoming and outgoing. It's not constantly draining all day. You might need someone to run to the bank to get a bunch of pennies or a bunch of singles to make change.
(And with the "cold storage" concept, it's free and instantaneous to move money into your own cold storage whenever your till gets over a certain amount.)