This has been done in the industry from very early on and is typically referred to as "Contextual" advertising. I and the company I co-founded are huge proponen

This has been done in the industry from very early on and is typically referred to as "Contextual" advertising. I and the company I co-founded are huge proponents of contextual as it generates positive ROAS (return on ad spend). The "problem" is that a contextual impression typically sells for less than an impression that is leveraging audience (user) data, so there is a tremendous incentive from middlemen to collect and peddle data - so much so that the entire industry all but forgot about contextual for almost a decade. I do think however that the promise of accuracy in audience data is very much oversold and not worth the tradeoff in user privacy, particularly in the loosely coupled world of programmatic advertising, which is anything in the open web and not something like walled garden (facebook) ads.