In the DT you are placing yourself at the mercy of every order before you. A delay to the front order at the DT in a peak time delays every car that's entered the queue. Maybe as many as 8 orders. So a delay of one minute there wastes 8 human minutes of life.
At the counter, on the other hand, it's not serial of course, so a delay to your individual order affects only you. So, even by old-fashioned good business sense (rather than modern MBA KPI-optimizing logic), it's reasonable to waste 2 minutes of your time, instead of one minute each of a whole queue at the DT.
(Yes, you can park the delayed car at the DT, but trust me, that's not a sustainable solution to do routinely. In general, getting "behind" during a lunch rush is an extremely tough and frustrating ordeal for the whole crew, no matter what)
Tell this to the idiots at supermarket checkout who wait for the total price to ring up before they reach into their purse (or whatever) to retrieve their checkbook and only then begin filling in alllllll the check's fields...
I didn't know it at the time, since it was my first job, but they got bonuses based on how fast they could make us work to fulfill those orders during certain times of the day. A backed up drive thru line would always get faster treatment than in store customers. Unless it was a special case, like a bus kid of field trip kids. I assume in those situations the managers could argue for their bonus based on the large amount of sales they got in store.
They obviously didn't share those bonuses with us.