[morei]

It's not quite true that advertising is a zero-sum game. There are economic benefits from increased competition, and from improvements in presented choice.

It is definitely true that the economic benefits don't accrue to the advertiser! And it's also true that the economic benefits decrease as advertising saturates. But it's still not zero.

[mbesto]

Let me rephrase because I don't think you understand what part of the advertising industry I'm referring to as zero-sum.

Advertising is basically cultivation of people's attention. There is a finite amount of eyeballs/"time of eyeballs" in any given any day for their attention. If all advertisers are trying to capture this attention, it means that for every advertiser who is able to capture an eyeball, it means another advertiser loses the ability to capture the eyeball (hence why Google/Facebook ads are auction based).