In theory you are right. Unfortunately ads tend to be for crappy products. The better a product is the less advertising it needs - the utopian side-effects free cure all cancer pills would need only a single CNN interview to take over the world. "One weird trick" ads need to run constantly because they are crappy.
Ads run up against human nature to want to share useful knowledge. If ads were really good we would have places to go to see ads.
They are also critical to the success of my business as we do a lot of advertising.