The simple answer is $$$ as the current infrastructure and equipment pools and crew and station facilities simply can't provide that type of service! While it would be great to have that type of setup, it will take at minimum a decade before any similar network can be put in service. If the funding came available tomorrow, and I were in charge of where it would go, there is already some work that I have done to prepare for some system such as that. My estimate of the cost of it for a 125 MPH nationwide network of new passenger only tracks is ~$30 Billion.
How did you calculate that estimate, if I'm correct Brightline in Florida is costing a few billion just for 125 MPH service for only a few miles, with most of it being 110 MPH and 79 for a portion. I would love if it only cost 30 billion, it could be financed completely by some of the rich folks like Warren Buffet if he was willing to put forward 30 billion dollars. But I think it would cost more than that, mostly due to the legal fees and controversy.