Ideally yeah, that would be the best but the supply and demand might not always match up. Maybe no advertiser is willing to spend some money on the content you're being served because it's very irrelevant to them or because there's simply no advertiser with enough budget to pick up the slack.

So you end up with highly valuable spots and then the rest which is at best barely enough to cover the costs of the publisher.

I'm not trying to blindly advocate for user targeted ads as the be all end all of online ads but the reality is that a huge swath of the publishers would probably disappear, consolidating the web even more.