NIMBY-ism as well? Because I can't fathom how a 400 miles high-speed rail route can cost $100bn unless you're spending years re-drafting your route because every asshole on the way has to be heard.
25 years is not completely nonsensical though, if you want to do this right it does take time.
For a fairly recent example, the french LGV Est is 252 miles (406km), it cost 4bn (including 800 millions worth of rolling stock), and took 16 years before the construction and financing protocol of the first phase was signed and the opening of the second phase. First phase opened a year late, and second phase 3 months late because of the Eckwersheim testing accident (https://en.wikipedia.org/wiki/Eckwersheim_derailment).
Obviously building in California would require additional studies and expenses for earthquake safety and the like, but 12x… seems a bit much.
The cost only makes sense if it's basically all tunnels and briges, like Chūō Shinkansen: that's estimated to cost north of $90 billion for 177 miles, but it's going straight through mountains, 90% of the way is through tunnels.