This is also why McDonald's introduced table service, which is only in restaurants that have a layout where it's impossible to hide how many people are waiting. It costs manpower to deliver food to tables, but the additional orders are worth it.
McD's don't mind if you have to wait, they mind if you leave before you order. "Busy-looking queue" is a much more frequent problem than "totally-packed-restaurant".
Source: I like hamburgers + I geek out over stuff like this. I.e., Just Guessing.