"they are just often hard to find in large deposits like other minerals.
The heavier ones are usually combined with Thorium which is mildly radioactive. The US classes Thorium as a radioactive hazardous waste while China is banking it for use when LFTR reactors or a similar technology is ready to use it. The waste disposal issue with Thorium is what makes Rare Earths less available in the market outside of Asia. The elements themselves are all over the place.