Why are these countries all mixed up in our SS programs? Yes, a little truth would be nice here.
There are reciprocal tax treaties.
US citizens who spend their working lives in the US can retire in England, for example, and they will be paid by the English pension system and a comparable Brit can retire here and be paid by SS.
I don't know how evenly that works out. I presume there is some money going back and forth at the sovereign level.
Tax treaties also address situations like Americans who do some overseas work. In most cases, to avoid double taxation, tax in one country is credited towards the tax of another.
So the "why" is because people are generally free to travel and some countries play nice.
Note, all I've said concerns LEGAL immigration only.
At least these countries are allies (except Canada ?? And Britain, in its inevitable future?). Unexpectedly, so many of these countries are in EUROPE, as are Freepers! What a wonderful outcome!