Never underestimate the ability of some President to waste tens of millions of dollars reaching the same conclusion after doing exactly what OP suggested.
At it's cancellation point, it was estimated to cost over $98.1 billion for a Los Angeles to San Francisco route - but we should note the cost estimates were going up $10+ billion every year they were re-estimated. The entire thing was supposed to "only" cost around $30 billion when introduced.
To make a High Speed Rail line that covers the entire United States... you're looking at hundreds of billions, up into the trillions of dollars... if not more.
At these cost rates, it's well over $200 billion for a straight line between San Francisco and New York City... but of course it won't be a straight line and it'll need to go to more places than those two cities in order to attract any sort of reasonable ridershare.
[1] https://en.wikipedia.org/wiki/California_High-Speed_Rail
However, I'm pretty sure that a large part of the costs of the California rail are related to private land, which probably isn't an issue through the majority of the route.
Compared to US where you have commute across the entire continent to leverage the largest pools and networks
This would be similar to commuting from Portugal to Estonia and thats not a thing.
When you factor in check in, security, getting to and from the airport which often isn’t near the city, etc, high speed rail is way better. Drop in 10 minutes before you need to leave and the stations are right in the middle of town. For distances under a thousand miles, rail can easily be faster.
I argue otherwise - cronyism and a whole lot of contractor bullshit continually inflated the cost as everyone tried to stick their hand into the infinite jam jar of taxpayer's money, until suddenly the whole thing broke apart and now nobody's getting paid.
One day I'll sit down and actually research this theory of mine - right now I'm only working off my experience with contractors in large construction projects (oil and gas) in the past. Their mentality - if it's tax funded, the amount of money you can get from it is limited only by your ability to bullshit.