I know that Japan is smaller than the USA. I also know that SOME major areas of the USA are totally covered with fiber optics AND cell towers.
What I also know is that none of those areas that are covered with fiber optics offer the speed and the lower cost internet that is available in other countries. Basically the customers in those areas have been paying to upgrade the infrastructure and now that it is there, the carriers are not offering them the best service they can at a lower cost.
You are right, it is all about the cost. Right now the major carriers are only interested in charging their customers the higher prices for sub-standard internet service and even when the infrastructure is in place, they have not lowered their service price or significantly increased the internet speeds.
But heck, if the customers accept it, why would they be interested in changing their business practices.