> What the ad networks want... What advertisers want is a real, credible KPI. When it's television, and you can show evidence that 2m people watch this progra

> What the ad networks want... What advertisers want is a real, credible KPI. When it's television, and you can show evidence that 2m people watch this program, so sponsoring gets you 2m of your target audience (or whatever you want to argue). With online, there are few web pages that get 2m people to read them in a day... and surely there's some effect from all those other sites out there. Some of them are even niche, so they might be a hard audience to get... Ad networks offer "all those other sites", and since you can't verify that the 50 people who went to that web page aren't included in the 20 people who read that other web page without "cross-domain tracking" you can't piggy-back on that argument being made in Television. Then you've got the second-screen phenomenon: Few people had two televisions in the same room, but almost everyone has at least two screens. Did I get this part of my target audience on their phone or on the desktop. Choices are, buy everyone twice (halving the advertising budget), or be a little more efficient: Ad networks know what they'd prefer, so they want that tracking to personally identify you now. These ad networks are the ones convincing advertisers they need a super mega cookie to track everything. And so on.