It’s a prisoner dilemma situation. It doesn’t matter how effective the ads are on an absolute scale, but a relative one. The aim is to get more facetime than your competition. Unfortunately, any company that opts out gets flattened.

Incidentally, this is why tobacco companies loved the ad ban (at least in the UK). It had long reached the point where they couldn’t encourage many people to smoke. They were advertising to cancel out the poaching of customers by other brands.