I think the point of "far apart" is the great emptiness between California's cities. In France or Japan, one train line services many many cities, right? In California, you might hit SF, LA, SD, SJ, SAC...
That's not necessarily terrible, because you can carry high speed through the empty stretches, but I think what it really says is any California train would be more like the shinkansen, which (while a great train line) is not as rosy as the other lines. Even in train-loving Japan, I believe it operates at a loss.
Here in Germany it is almost comical how smaller cities fight to be on the high-speed ICE train network. But not all trains are high-speed. There are lots of slower speed trains which reach around 200km/h. The ICE are supposed to connect the larger cities. The ICE from Hamburg to Berlin has a fast connection which allows people to commute over that distance - something which was unthinkable with slower trains. But there are also cheaper and slower trains, which are servicing more cities between Hamburg and Berlin.
That's not necessarily terrible, because you can carry high speed through the empty stretches, but I think what it really says is any California train would be more like the shinkansen, which (while a great train line) is not as rosy as the other lines. Even in train-loving Japan, I believe it operates at a loss.