The problem is that you have no idea if those ads are working, not really.

Do I go to McDonalds because I saw an ad, or because it 3 minutes away and the kid wants nuggets? Do I shop at my local supermarket because they buy every third ad on YouTube, or because it's the closest? There are a ton of details that influence our daily choices, and I'm not suggesting that ads aren't one of them, but I'm starting to doubt that it's the defining one. When I look at what I buy it's: availability, cost, and specification. There's not a hold lot advertising can do about any of those three.

If we could truly measure which ads works and which doesn't the entire ad based economy would crash over night.