That only works if you actually click and buy. If you see an ad for a part you need, then go to hardware store and buy it they have no way of knowing that you saw the ad, much less how much influence the ad had.

It also fails to account for awareness being a major reason to buy ads. Ford want to advertise to everybody (probably including Amish) because most people will eventually buy a car, and it is important that you think Ford then. Even if you bought a car yesterday it is important to them that you not forget that you can buy a Ford - they know you won't buy for a few years, but they still want you to think of them.