Density over some wide area is the wrong metric here. An entire state is absurdly large (in most cases). Two dense endpoints can be separated by quite a distance and still be worth connecting.
Tokyo has 8.9M people while LA has 3.9M people; half the number. Osaka (3rd largest) is 2.7M people while San Jose (3rd largest) is just under 1M people; again, half the number. The distance from Tokyo to Osaka is 500km, while from LA to San Jose is 550km. (In both cases, the 2nd largest city is nearby the largest.)
You have half the justification in CA you do in Japan, looking at similar urban areas. In the case of Japan, connecting those two cities goes through another 3 with over 1M population (and totaling over 7M together).
US cities are spread out and relatively low population.
> Tokyo has 8.9M people while LA has 3.9M people; half the number. Osaka (3rd largest) is 2.7M people while San Jose (3rd largest) is just under 1M people; again, half the number. The distance from Tokyo to Osaka is 500km, while from LA to San Jose is 550km. (In both cases, the 2nd largest city is nearby the largest.)
Paris has 2M people and Bordeaux has 1M people (sixth largest urban area in France), they are 500 km apart. How come France is able to operate a dedicated high-speed line between these cities? The third largest city on that line is Tours with 360k people or so.
California has almost the same population density as France, actually, but it's less spread out so just one line would be much more useful than a single line is in France.
Canada just announced a high-speed line project between Québec, Montreal and Toronto, by the way.
Why is Japan's density the magic number? A given route is either viable or not.
The US historically had much more passenger rail than it currently does. You haven't provided any convincing reasons why that shouldn no longer be workable in the modern day.
