I'm sorry, I don't understand. Are you saying that there exists direct advertising libraries for web development frameworks and CMSes, and that they've possibly been around for a long time?
Sorry for the slow reply. I'm not familiar with currently available frameworks/etc. so I can't speak to that side.
The point I intended to make is that, in my experience, there aren't site owners going "I wish I could do this if only it wasn't so technically hard", they're going "I wish I could do this if only it didn't mean people would spend less money with me". Because on the other side of the equation, ad buyers (generally, though obviously not universally) don't want to work that way, and they're the ones with the money and therefore the power.
I'm not a developer myself but I've worked with developers who created various in-house ad-tech, and while this sort of thing may be too complicated to do for Mr Random Joe Blogger without a CMS plugin, it's really not something that is too technically complicated to be feasible.