Rare Earths Are Rare in the US Only Because We Choose To Export Environmental Challenges
As has been said by many commentators, rare earths are not particularly rare. Via source, here is an estimate of their abundance in the Earth's surface:
Note by the way the Y-axis is logarithmic so small changes in vertical position can mean a factor of 10 or more difference in concentration. But the rare earths are not unreasonably far off fairly common industrial metals like lead, nickel, copper, and molybdenum and well more common than gold, silver, and mercury.
I am not a geologist but my gut feel is that there are plenty of rare earths in the US. The problem is that the process of mining and concentrating the minerals is ecologically relatively expensive -- often large strip mines are involved with a lot of tailings and waste and sometimes dangerous chemicals used in the concentration process. By the time some of these materials starting coming in demand, most companies had written off the US as a viable place to even look -- why bother exploring if you are never going to be permitted to produce anything. Thus much of the world's production has been outsourced to countries like China and poor nations in Africa (or the worst of all worlds -- Chinese mines in Africa) that don't necessarily give a sh*t about the environment.
From an environmental standpoint, this is actually a terrible situation. The US has more wealth and a fair amount of will to take environmentally sensible approaches, so exporting the nasty stuff is not a great long-term solution. California by the way is the worst about this, intent on kicking out everything from coal-fired electricity generation to oil refining from the state but happy to buy the products of these industries from less environmentally-sensitive areas.