I used to think the same thing, then met a guy who was a fairly prolific ad spammer. He basically said the reason any type of adware exists is because it actually works. He said if nobody ever bought anything or clicked on any of the annoying pop-ups, adware would disappear inside of six months.
In essence, for web savy users, it's completely baffling why people would want this on their PC's. In reality, there are a ton, and I mean a TON of people who do click on the links and do buy from these types of invasive ads, which continue to be a very lucrative, very competitive market.