> why is it so assumed that only China can produce rare earths?

Pollution. The smokestack emissions are very toxic. The residue/slag is toxic and radioactive. One should remember that "rare earth metals" are not rare, they're the bottom 2 rows of the periodic table. They are rather hard to separate chemically and many people like to exclude the bottom row of the periodic table (the actinides) because that's where uranium and plutonium are located and those 2 elements terrify people enough to derail discussions about the materials.