Generally speaking, private companies want to make money by getting customers. Obviously there can be edge cases, but if there's profit to be made by hooking people up, they'll want to do it, and if private companies don't want to get more customers, you have to ask yourself some hard questions about why.

I think we both know what's usually happening: people in an area who, as a whole, are rural enough and poor enough that the economics don't really pen out well. And I'm sure said corporations would be happy for the local government to pay the cost of running those lines out -- if that's not happening, ask yourself why those local governments don't want to pay for it either.

Now if you want to say, "well I don't care if it scales badly, the federal government should just subsidize it until it works", that's your prerogative. But another option would be to encourage zoning and similar rules that impact how people live to change towards better scaling of infrastructure and services, so that spending on these kinds of things is more sustainable and fair.