I agree that as an aggregate we're probably massively overpaying for ads for little average effect, but there may be a game-theoretic reason why it does not go away. What if out of 100 competitors the top one gains a lot from ads but the 99 others lose? All 100 have to pay to play to try to be that one. If they cooperated as a cartel they could all pay less for ads and consumers could pay less for products but that is unlikely to happen.