Google ads used to be valuable because they were based on the content of the site you were currently watching. Doubleclick (later bought by Google) and others already did the tracking thing and didn't get mileage out of it. Retargeting (i.e. following you around with ads for something that you may buy with a more than 3% chance because you actually searched for it) is the opposite of that, purely tracking-based and mostly annoying.