I talk about this with my wife once in awhile since it seems to happen a lot more then I remember in the past. My theory is that with the advent of dashboards that are always backlit, even with headlights off, people don't realize that their headlights are off. It can be a bit hard to tell when your interior is lit up and you're driving in an area with lot of ambient light. Most cars have auto-headlights and people just assume they came on and can't really tell that they're not on. So when you flash them they have no idea what the issue is.