The explanation I've seen is that yes, most people who buy certain products (a microwave oven, say) are probably done for a long time and less likely than the average person to buy one, but overall the group of people who just bought a microwave oven are still much more likely than the average person to buy another one. So yeah there are a bunch of people for whom the ads seem extremely stupid because of course you don't need another one, but enough who think "you know, I could use one in (the camper/the basement/the rental/the garage-office), actually, but it'll need to be a smaller one" that it's worth advertising other options to the whole group. (for the record, I hate all ads and wish they'd go away)