Most sites are not in business of running ad marketplace. They are in content production and SEO(as in pulling views). I don't think at scale they could operate it would be worth to be ad marketplace. Or would they get the customers(advertisers).
> Most sites are not in business of running ad marketplace.
You don't have to run a marketplace. Back then, people were calling via phone to put an ad into the newspaper. The same concept can still apply today.
You can just place an ad _where you actually want it_ instead of having a shitty decision engine decide it for you. Marketing budgets are messed up all over the place since social media, because they're really inefficient in finding the customers that actually wants to buy their stuff.
If you don't believe me, just take a look at facebook, they're trying to shove ads up your feed 99% of the time. It's a feed of ads, not a feed of "friends" anymore. Good luck trying to catch the 1% of posts that were actually the reason why you used the platform in the first place.
Content based ads work fine for this. If you have a site about some topic, advertisers for that audience can just pay you some amount to put their ad in your posts or embed an image.
Networks are only necessary if you want to run new ads for every year or tie into their browsing history or other algorithmic stuff. But if someone wants to pay you 3k to run a banner ad for a few weeks that would be pretty easy to just embed.