The Ad matching service gets paid when you buy something, regardless of whether it is relevant, useful or needed by you. What is effective for you, and effective for the ad network are two separate things. Also, ads by their very nature are designed to manipulate you into buying a product. The more personal information they have, the easier it is to manipulate you. Maybe you're still OK with that, but many people think that this is the wrong direction for the web.
Oh no, nobody depends on linking up an ad impression with an actual purchase. Not only is that incredibly hard to do, with few exceptions, the conversion rate is abysmal.
Money changes hands at the time the advertiser agrees to buy the space on the publisher, based on the CPM of the offer. A publisher that can more reliably link ad impression with sales can charge a higher CPM, but nobody in their right mind would buy and sell ads based only on conversion.
