> now there's better tech to do native ads without the intrusion and massive bad experience .... the publisher themselves can serve you a small ad

This has always been possible and, from my recollections, was how some early online advertising was done. (I don't know if it was ever the dominant model, but it was certainly more common.)

I suspect the difficulty connecting content providers with advertisers is the root of them problem. After that, it takes effort to maintain that relationship. Having a third party handle everything is simply easier, especially for smaller web sites and smaller companies who want to advertise.