They earn huge amounts with a 5 million won tax exemption on merchant ships, so why don't more people join?

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A 5 million won tax exemption for foreign vessels? That is massive... Usually, working abroad only offers a 1 million won exemption, but
people on ships get 5 million won tax-free? Wow. Despite that, why don't more people do it?
I heard promotions are fast because so few Koreans join, and they say making 100 million won happens in the blink of an eye.
And what's shocking is that contract workers, not permanent staff, work on ships for over 6 months, come ashore to collect unemployment benefits for 4 months, and then go back to sea;
they repeat this cycle. It seems like a sweet deal since they get to relax while receiving unemployment benefits, yet they say there is a shortage of Korean sailors;;;
If I could go back to being 20, I would have gone for the "merchant ship tech" route—working on ships for about 10 years before getting off.