> Is there a reason more sites don't do self hosted ads?
Google Adsense (for example) is a live auction clearing house where you paste a snippet of code into your site that resolves into the highest bidder for the keywords on each page. You're done.
Meanwhile, hosting your own ads entails finding advertisers, wooing them, developing a relationship with them, maintaining that relationship, negotiating a contract, settling that contract, building an ad-serving system somehow, automating it somehow, etc. And that's if you're even big enough to attract anyone or even warrant the work to begin with. And the second you try to generalize it (which is what ad networks do), then you're back on every adblocker's default blocklist.
Meanwhile, hosting your own ads entails finding advertisers, wooing them, developing a relationship with them, maintaining that relationship, negotiating a contract, settling that contract, building an ad-serving system somehow, automating it somehow, etc. And that's if you're even big enough to attract anyone or even warrant the work to begin with. And the second you try to generalize it (which is what ad networks do), then you're back on every adblocker's default blocklist.