So, if you believe in this idea so strongly, create it yourself.
Just remember that you have to take into account little things like:
- fraud prevention
- interfacing with wildly varying ad servers with wildly different features and wildly different conventions for ad calls
- the need to pull inappropriate ads from the cached ad queue asynchronously
- proper reporting of ads actually delivered
- proper ad rotation, which will depend on ads have actually been delivered
- handling rotation limited ad deliveries
- different webserver environments (IIS, apache, yadda, yadda, yadda.)
- and many, many more...
Can most of this be done? Actually it is possible, although I suspect that the resulting adserver would be feature limited (remember, it needs to work across all kinds of different webservers) and there are some features that cannot be implemented without a central adserver to coordinate the data.) The end result might be something that is too complex for websites to actually implement.
One of the biggest barriers would be the website owners. They don't want to mess with their servers just so they can deliver ads. Contrast installing something to manage the cached delivery of ads pulled from some ad providers adserver vs doing something like adding two lines of javascript (one to load the ad delivery library and one to make an actual ad call.
It all seems simple until you actually have to implement it.