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Advertising is much more than information. Advertising is all about image. The best advertising doesn't actually tell you anything about the product beyond the impression that you will be viewed as cool and interesting to the opposite sex and your peer group if you use it.

This is why Pepsi and Coke cannot stop advertising for even a second. It is even more important for them than for other manufacturers in other product areas, as Coke and Pepsi are so ridiculously similar to one another. They must not only constantly reinforce their image to their current group of customers, they must continuously appeal to new ones. You and I may be very familiar with the products, but kids are being born all the time.

Also, reducing your advertising by half isn't nearly as cost-effective as one may think, as they would lose some (maybe a lot) of their volume discounts with the media they currently use. The 12x rate is much cheaper than the 6x rate in a magazine, and I must assume that the same goes for radio and TV. To reduce impact by 50% to save 30% isn't good math.

Also, any reduction must be matched by the competition, as a one-sided reduction in advertising would result in an increase in market share by the competitor that continues to saturate the media with their message.