With a webpage you contact a server and request for the data to be sent to just you.
With TV the data is broadcast to everyone at all times and they sell you hardware to decode the signal. They have no way of knowing if your hardware is turned on, or what settings it is turned to. Now, with things like TiVO they may be getting some data back, but a "converter box" was specifically mentioned, and those are NOT smart. Every channel is being sent on the wire coming into your house at all times.
No, they aren't likely to suggest that you turn on a radio. And yes, they should. But that would be admitting that you can get just as good information elsewhere, which might affect their revenue in the long term. But it's about four levels removed from directly affecting their bottom line. First, you'd have to have someone that only ever thought of TV to begin thinking of something else. Then they would have to do it. Then they would have to participate in a Neilsen rating. Then the advertisers would have to negotiate for a lower rate based on lower ratings.
That's what the industry goes by, and what advertising rates are determined by. The only devices they have go in *select* homes. I've been asked to participate once in my life, it's not something that a random person is likely to have ever actually done, therefore their watching patterns don't (directly) affect anything. (No, I didn't do it).