Because there are people who buy more than one of something.

Buy a widget - you might be purchasing for a company and will buy a the same widget next month for the next employee. If you advertise your widget to everybody 99.999% won't buy it, so that 99.9% that won't buy another widget is a much larger margin of those who will buy another. (the ad is about either keeping you a loyal customer or getting to to switch to a different brand). Most of us are more valuable as a possibly purchaser of more than one than as a potential customer of something new we didn't even know we needed.