The reason why China is the source of all of our rare-earth metals is because they are aggressively mining & processing it. There are a variety of reasons why China is doing the mining and processing, including factors like proximity to supply chains, China's strategic goals, different environmental & labor laws, land use laws, etc.
As soon as it gets more expensive to source rare-earth metals from China, other sources will come online. Other countries could ramp up mining & processing, perhaps as a "matter of national security", but for whatever reason that hasn't happened yet. This is done in other areas, there are a fair number of industries that only exist in the US because the military-industrial complex is interested in keeping supply chains independent of influence by foreign powers.