We let China do it because the refining of rare earths is incredibly dirty and environmentally destructive. The ‘leave behinds” include toxic soils, radioactive materials, heavy metals, and other pollutants rending soil unusable for generations and air polluted for miles.
We were at a point where we cared about the environment when rare earths became important, and we were happy to let someone else deal with the mess. Now we have decided that we need to be in better control of our own destiny, so we’re willing to try to find ways to do it ourselves. It wasn’t “cost”, it was “environmental expense.”