> If they could somehow target ads based on who's watching, the ads would be massively more effective

They already can, and do. They know what the audience of their shows is and schedule the ads accordingly. Don’t show ads for funeral homes during Saturday morning cartoons. Hell, this is how we got the whole concept of a soap opera: soap ads during shows targeted specifically at housewives.

You advertise cars during car shows, tools during DIY shows, toys during cartoons, etc.