The main reason this has not happened is that it requires the ad networks to trust the publisher on things like visits/impressions/clicks. If your server has t

The main reason this has not happened is that it requires the ad networks to trust the publisher on things like visits/impressions/clicks. If your server has the ability to say "Hey, Jane doe saw your ad just now" when she didnt, thats a recipe for massive fraud. You might say CPC would work and then just send to unique urls the ad network can track- This too wont work, as it allows the publisher to do things like auto-pop the link, hide it in 1x1 iframes, etc.. Overall its a security and fraud nightmare that involves huge cat-and-mouse games. Not to mention the fact that the ad networks have no incentive to change. As more inventory drys up, the spends go up with them (competition increases) - Supply and demand, mostly.