The biggest reason is that most publishers completely lack the ability to set up and manage an ad server. Next would probably be that the advertisers, fearing

The biggest reason is that most publishers completely lack the ability to set up and manage an ad server. Next would probably be that the advertisers, fearing fraud, want the publishers to use a third-party ad server with reporting that they trust. Only after that comes concerns about behavioral targeting and tracking, and the absence of that affecting ad revenue. Finally, if they thought about it, they'd realize that ad blockers would just switch strategies and carry on blocking, so it hardly seems worth the effort.