It makes a lot of sense and I'm sure advertisers would very much like it as much as you do (in this specific context that is).

I mean, there's a reason this infamous quote is still relevant today: “Half the money I spend on advertising is wasted; the trouble is, I don’t know which half.”

The real issue is the ad-ecosystem is afraid (and rightly so if I may) that people might try to game the system and would rather not bother.

That and realising that out of your X thousands / millions monthly unique visitors, only a small percentage are an adequate audience will pelase neither the publisher nor the ad exchanges. Sure you could fall back to branding adverts but those are usually of lesser value.