For the same reason CNN take three "pages" for their story: They don't understand that the world has changed. Scrub that: They don't understand the world. They really don't.
Here in the UK we have the wonderful BBC. I can watch everything the BBC produces online in real-time and for the following two weeks (or something like that). All radio stations are also available (and retained for weeks). It's brilliant, and I love it.
I don't own a tv -- well, I technically have one, hooked up to a PS3 in the playroom -- but it has no tv abilities. Actually, that's not true either as I can access the BBC's iPlayer via the PS3, but that's not the tv's fault.
The only reason folk can't watch tv online -- presumably in the USA -- is because the "transmitting" stations don't provide the service. That's idiotic.
This is clueless, narrow-minded marketing. But I'm used to that now. I place marketing one below estate agents in the imagination stakes. And they've earned that place due to utter incompetence over the past decade or two.
In the real world, there is no reason why this can't happen. In the real world, it's happening all the time, 24/7. It's happening right now.