The complete boxes come from brinks and often times you can get rolls people bring in by asking for fewer than a box full. It seems to bother the tellers having to count out all the rolls as opposed to getting a box.
Either way its a hassle to the teller BUT theres a difference between interacting with the tellers and actually getting to know them, instead of just asking for the box and leaving. I know for my bank the teller has to get someone from the floor *I'm assuming a manager* to get a key to go into the safe.