$300-500 billion is a lot to install a national fiber network because "we might as well". If they're going to cost the same to operate, what do we get in exchange for this $500 billion investment? The ability to watch Netflix videos at a higher quality? A slightly lower cable bill? (since the cost to maintain won't be significantly lower, the bill won't be either)
I'm not trying to be sarcastic here -- this is how the government thinks about things. We have an existing infrastructure that is less than optimal, but it still functions and is in no danger of imminent collapse. Contrast with our healthcare system which barely functions and is on the verge of effective collapse. Or our public schools and police departments, which have collapsed in a number of cities.
In short, it's hard to drum up political support for large infrastructure projects even if you can show the need. There has to be a demonstrable catastrophic impact if we don't do the project, which just isn't the case with broadband.
We already have physical networks. Maintaining them is a fixed cost.[1] You can't eliminate it because you can't reasonably expect to replace the entirety of Cable TV and U-verse with cellular, so the money might as well be going to maintain fiber rather than copper.
[1] http://en.wikipedia.org/wiki/Fixed_cost