'should serve the ads themselves'
One interesting aspect of this - speaking generally about the online publishing world - is that no one believes each other.
If a vendor goes direct to a B2B publication, books an ad campaign, pays $20,000 for 300,000 ad impressions, and then after a few days or weeks, the publication says, 'ta-da, all done, thanks for the money,' that vendor may be skeptical the ads were actually all served to real humans. Especially when the ads are targeted and we're in an era of ad-blocking.
So the advertiser wants in on the analytics to verify the campaign was truly fulfilled and/or to see it progress in real time. The ads in that case end up being served by or involve a third-party both the advertiser and the publication trust / willing to use.
I'm simplifying here to avoid talking about specific operations, and we're happy with our ad clients, and our ad clients are happy with us, of course. My point is, the industry is a very very long way from placing a banner ad on a page and letting it run its course. It's more complex than most may think.
C.