They don't exist anymore though. According to Australia, all of the rare earth expertise is in China now, so it's not as simple as just reopening, and in fact might be very difficult
This is a changeable situation. Expertise can be obtained. We have plenty of experts in Engineering, physics and chemistry. Everything about mining and processing comes down to those. Experts in the field know exactly what is important to know and and won't try something that seems good but doesn't work out. However there is no reason a few years of investment can't create experts of our own from the basic principals we already have.
Note that we have experts in mining and refining scattered around the country. While the specifics of rare earths are different from iron or gold, many details are close enough for round one, from there is is just improving the processes.
The US Mountain Pass mine in California is operational. They send all of the output to China for processing. That will change in a few years as the US brings new domestic processing facilities online. The US chemical company, Blue Line Corp, has partnered with Australia's Lynas to build a facility in Texas. They're forecasting two years from now to have it running. That facility should enable the most critical / sensitive domestic demand to become entirely independent of China.
Note that we have experts in mining and refining scattered around the country. While the specifics of rare earths are different from iron or gold, many details are close enough for round one, from there is is just improving the processes.