South Korea is smaller than my state of Missouri. Now let's see how fast the US can provide broadband speeds if they dumped all their resources into my state alone.
Why do you think google fiber is so selective? Because it is a regulatory nightmare for google to get the unbelievable amount of paperwork done for approval. Even for cities that want to work with google, a lot of them still aren't really able to due to the amount of bureaucracy and red tape required to pass planning/zoning committee muster for each county/city/etc.