“It's a security thing after hours. It's a tiny bit harder for someone in a car to hop though the drive through window and rob the place and if they do, someone who comes later might get nosy when no one answers the speaker but there's a car idling in drive through for 10 minutes. At least, that's how McDonald's literature explained back when I did a summer there in the 90s. Although I never saw much point. All they'd need is an accomplice to bypass that easily and we always broke the rule for truck/RV drivers anyway.
Although keeping inattentive drivers from mowing down people standing in drive through is a pretty good reason and is probably just as much a part of it.”