Rare earth and Propellant are considered natural resources. Yes, you definitely want to get natural resources from others , just like Oil. They are not related to made parts.
Only part correct here. Yes.... most countries with oil reserve would still prefer to get natural resource from others... why? Because of a couple of things...
1) Cheaper - so it still boils down - ta daaa... economic.
2) Some of the oil reserve in that country are much more difficult to extract.
3) Don't have the technology to extract the oil
4) Political reason (Green movement, letting other deplete their oil so they can be the last one still standing with oil, etc)
It is exactly like made parts. It is business and that is all to it. And might I remind you... US is moving back to extracting oil from their own country now, because they might find it cheaper and faster that way.