Just as the parent poster says, the U.S. chooses not to maintain its infrastructure. The infrastructure is not affordable by many municipalities, but that same infrastructure is affordable by U.S. as a whole.

They could subsidize the infrastructure required to integrate the country and keep the non-dense areas viable even if these areas don't have the local tax base to pay for that infrastructure, just as they did with federal highway dollars when they were built, just as many other countries are doing. It's a choice to do so or not. The rules of who pays for maintenance of which parts of infrastructure are essentially arbitrary and the U.S. can alter them if they choose to.