FDW
Banned
You might have something of a point for upgraded conventional service like Jetrain, but in terms of true HSR while what you say is true, the benefits are marginal and the costs very high. Basically, with no on line traffic the actual cost effective of HSR starts to fall apart pretty quickly in most cases while at the same time the trains typically accelerate fast enough that savings from small reductions in the number of stops aren't all that dramatic.
The more significant issue that for most of the country population density really isn't that low. California for instance has a density fairly equivalent to Spain's, and is only really proposing a single HSR line for it. The east and midwest are largely similar, in that while they don't have some of the really dramatically high densities of specific parts of Europe they aren't exactly what you can call wastelands either. No, HSR isn't going to work coast to coast, or for all areas, especially in the west, but for a good deal of the country it really isn't fair to say that density is all that low.
Correction, Mountain West. A Portland-Sacramento HSR corridor would be perfectly feasible, if expensive.