It's like the whole purpose of ads, they're not trying to sell you stuff you need - they want your money.
Google wants the advertizers' money, who are really being scammed by Google if they pay every time Google serves an ad to me that Google presumably knows is not going to result in a purchase. I guess Google don't care - if they get paid for click-thrus and I occasionally do that, but never buy, then Google still gets their money.