Engineer here.

Basically, it's all about workflow. With the drive through, you've got this system that is very linear. The result of that is a flow that can get incredibly backed up easily if one step takes a long time. And believe it it not, placing the order is often the slowest step. This is why many McDonald's have put in 2 drive through lanes (while still keeping just one payment window and another pick up window which all cars must funnel through).

This basically allows more orders per minute to be collected, as well as preventing clogs due to grandma taking her goddamn sweet time to put in her order