Headlight optional?

So what is it that causes people to not replace their burnt out vehicle headlights? Every day that I am on the road I see cars with a burnt out headlight. So do people actually realize when they have a burnt out headlight? I know that I sure would notice. Especially when it is the driver side headlight. But I guess that some just don't quite get it. Perhaps if they were taught how to use their eyes, they may be alright.