In other forms of advertising, the advertisor is forced to confront the fact that the connection between advertising and purchasing is not a direct process because they can't see it anyways. They can see that they advertise and somewhere down the line they can observe a general rise in sales or brand knowledge, but they can almost never tie that to even a campaign, let alone a particular impression.

So you have this market for advertising where the advertisers can point to abysmal CTRs and say "See? I shouldn't be paying this much for such terrible action!" while still reaping the benefits of less tangible brand recognition down the road. And thus is born a race to the bottom.