Why don't the main networks (ABD, CBS, FOX, NBC) have live feeds online?

Question

Since the local affiliates send out signals that can be picked up free with 'rabbit ear' antennas, why don't they have a local live feed online? It seems like they are missing some major advertising opportunities. For example, when I am sitting at my office downtown, I don't have access to television. But if the local CBS network had a live feed of their normal programming, I could watch online. Or I could go down to the break room and watch the same show on the television, just by having an antenna hooked to it. It seems like it doesn't make a difference.
I can understand why cable channels won't do live feeds... they need the revenue from cable subscribers. But the network channels make no money from cable subscribers... they make money from advertising that is shown whether you have cable or not.
Any clarity on this would be helpful.

One answer was a bit helpful (thanks), but I think there is a huge market for people that want to watch online. For example, I have 2 offices (one at home and one at work) where I do not have a TV. I would love to work late at night and have a little window on showing Monday night football (or something like that). I cannot watch it while at the office, so they are losing me as a viewer.
Also, live feeds don't take up the same amount of bandwidth as individual interactive feeds. If they are showing one program, and people don't have the option of fast-forwarding or pausing, then it is pretty simple to send the feed.
I can imagine there would be tens or hundreds of thousands of people that would watch programs online.

Answer 1

Because the bandwidth costs wouldn't justify such an endeavor. They've probably already figured out how to maximize revenue from their web properties, so why bother adding something that will cost money to implement and won't really generate much more money as far as ad revenues. I'm pretty sure their own market studies have revealed that most people are not accustomed to watching TV shows on their computer. Most people prefer the TV.