If you dig a little deeper below the surface (excuse the pun), you'll find that while they have the label "rare earth metal" the majority aren't actually that rare. They can be extracted in lots of other locations, but they are often discarded byproducts of different mining operations. In order to extract the rare elements you need to build extensive refining systems which require a lot of capital upfront. China created these processes and scaled them up massively, but they are replicable elsewhere. The only reason other countries did not both was because they can be rather messy, toxic and China was previously a pretty reliable supplier of raw ingredients - but now they're stuck in a trade war they've decided to restrict the flow