They could address that by going the other way. Instead of serving the ads from the content provider's server, serve the content from the ad provider's server.
Essentially, the ad providers would also become hosting services.
Many content providers won't like handing over as much control as such a situation may imply though.
It also creates single points of failure that did not exist before. If the ad service is down, your content is (potentially) down too rather than just being served without working ads.
Essentially, the ad providers would also become hosting services.