Korea does have good healthcare. But most of the research, technology, and innovation occurs in the USA. One of the many things the USA subsidizes in the world.
The healthcare is "cheaper" because the government heavily subsidizes it and pays healthcare workers much less. I have a few friends who are physicians there and they tell me a lot about the problems they have (low pay, medical errors related to long hours, alcohol, etc.). Korea also has a much higher tax rate and cost of living compared to the USA.
Another reason why healthcare in USA is so expensive is because we too many lawyers who like to sue. So physicians practice defensive medicine that significantly adds to the cost. Korea and those other countries don't have to deal with that:
http://www.massmed.org/Advocacy/Key-Issues/Professional-Liability/MMS-Study-Puts-$1-4-Billion-Price-Tag-on-Defensive-Medicine-in-Massachusetts/
Those healthcare outcome rankings are misleading as well because they don't take genetics, culture, behavioral, and other factors into account.
https://www.wsj.com/articles/SB125608054324397621