That's because at a certain point ads become less about direct sales, and more about brand recognition. This is why Coke has a massive advertising budget. You know what Coke is. You're either going to buy a Coke or you're not. But by advertising as heavily as they do, they're reminding you that they exist.

That's why their heaviest advertising season is Christmas. Some of the advertisements don't even show their product. Some do, but some just show Santa flying his sleigh with cheery christmas music. Then their Coke logo gets spelled out in the stars as he passes by.

It's just so you assosiate their brand with good emotions. But it requires an almost constant barrage of ads, and it's direct sales are impossible to measure.