There's a lot of good comments here, but folks are overlooking a major structural issue in the USA that IMO is largely at the root of why rail development has become so complicated - and that's the fact that local jurisdictions are very powerful compared to many other countries. Every city and county along a route (or even potential route) gets a seat at the table, and will work to represent their constituents and economic interests. This means there's a ton of voices advocating for completing priorities - they want rail to go through their city or county, but only if the HSR gets a stop at their preferred locations. If the jurisdiction doesn't get what they want, they'll often work to grenade the entire project.

China, by comparison and for example, has an authoritarian central government that can just dictate a route, displace individuals or communities in the way, and tell anyone upset to pound sand.

Not trying to say that authoritarianism is the answer. We're well on our way to that, and it's sure not helping things. It's just a structural difference in the USA that makes makes such projects far more complicated, expensive, and inefficient.