There are other rare-earth deposits in the US, at least two in Wyoming and one in Arizona.[1] A different group claims to have a big find in Colorado. The Wyoming people claim to have found a deposit so big that if they built a large mine, they'd crash the rare earths market. Those are all in exploration stage, with no production.

MP Minerals was mining ore and shipping beneficiated ore to China for further refining. So it made sense from a DoD national security perspective to fund on site refining in California. DoD only funded one of the four steps from mine to magnets.

[1] https://cowboystatedaily.com/2024/02/07/rare-earths-discover...