They already do this if possible. The VAST majority of publishers are selling only a small percentage of their own ad space (the giants of the industry might be different, but even they probably have third party networks to fill gaps in their inventory). Instead, they partner with networks who auction ads off to the highest bidder. That bidder is usually going to be a company who trusts the ad network to track users and show ads relevant to either the interests they've shown or the current content.

I work with a publisher on these sorts of things (I am not an ad network, though). Some of those networks have strict prohibitions against showing ads on pages featuring certain types of content, and so we've had to build our own system to display ads on those pages which work with networks which are fine with that content. The publisher does sell its own ad spaces to interested local parties, but these are usually accompanied by periodical ad sales because the company ultimately makes more money selling ads through networks.