"Labor shortage" in the context of high unemployment (edit: as in South Korea, not Japan) usually just means, from the point of view of employers, a shortage of suckers who are willing to accept low wages.

Besides, the birth rate in SK hovered between 1.4 and 1.5 during much of the 80s and 90s, and only dropped to current levels (around 1.1) since the early 2000s. Most people who are looking for jobs right now were born in the 90s; the ultra-low-birth-rate generation is still in K-12. It's a lame excuse.

Foreign workers are willing to accept the kind of crappy treatment that most young Koreans would not hesitate to report to the authorities. That's why they're hired and Koreans are not. Then the Koreans become foreign workers in Japan...