I mean, they allow for much, much more efficent transit space-wise in high-usage routes? (I was thinking more of light/commuter rail, which it seemed like you also took issue with)
If you're going to a place that's dense, car storage and mobility are a nightmare. I have family that worked in SF. To go to work cost $25 a day before factoring in gas and wear and tear. And I lived about 7.4 miles from the close end of the bridge. Some folks come from farther than that. Light rail is a tremendous advantage there. In some spots it's just a park-and-ride arrangement that skips traffic and fosters growth.
I don't totally know the full ins and outs of HSR, so I can't speak on it directly. It seems like there are certain spots it would make sense if the geography allowed. Something like Madison to Milwaukee (that might be too hilly). The kind of thing where 75 miles or so to expand commuting range. SF to San Jose is I think not geographically feasible (too much in the way maybe?).
Public transit is effectively a density problem. If the density is high enough, public transit makes a great deal of sense.
I find that "light rail" tends to be pushed in tons of places that it makes no sense, though, and actually causes economic harm. Light rail is tremendously expensive, and once it's built it needs to be maintained. That takes a lot of money. Ridership on most light rail systems does not cover the cost of building/maintaining/operating the infrastructure, so it has to come from somewhere.
You know where that money often comes from? Buses. Bus lines are pared back to save money that can be used to keep the light rail afloat.
Which means that in a lot of places,
you end up hurting the people who need transit the most (those who can't afford anything else) and ride the bus, in order to provide rich suburbanites subsidized light rail.
The other argument to me is that flexible infrastructure is better than fixed infrastructure in a lot of ways. Let's say we build HSR from SoCal to NorCal. But because of the taxes needed to operate that HSR, more and more tech companies--as is being predicted here--leave Silicon Valley for places like Austin. Daily weekday trips between SoCal and San Jose drop by 15%, and daily trips between SoCal and Austin go up 10%.
For airlines, that's an easy problem to solve. Just fly less planes daily to San Jose and more planes to Austin. For rail, there's no answer. You just lose more money.
Airplanes and buses are flexible infrastructure. HSR and light rail are fixed infrastructure. We seem to keep wanting to build the latter
hoping that the economics will work out, but hope ain't a winning strategy.