TV Providers (we called them broadcasters in my day) have no idea how many people are watching. They relied on the guess work from Nielsen. So I'm not really sure what you're implying they can do.
It wasn't until digital cable boxes before they could start tracking your account watched what channel at what time on what box. Or at least what was being decoded on that device whether eyeballs saw it or not.
In other words, you're focused on entirely the wrong thing if you're being pedantic with "nor do the web ads themselves". Thanks, as if I was confused on the subject. We're all adults here and understand the video bits are not tracking but the code used to deliver them.