Australia

It was nine years in Australia and married and with PR. Not all of those things happen at the same time.

But regardless, there can be very good reasons why Koreans living in Australia may wait longer to give up their citizenship. Australia has awful (basically non-existent) public dental care that practically forces anyone other than those with a government health care card (who have a very long wait to be seen), regardless of income, to pay for private dental (or just ignore their oral health). Korea has universal dental care that is very affordable and of a high standard. This is particularly important for those who have more serious dental issues from childhood or genetics.

If you have sick or older relatives back home, the freedom to travel back and stay for a long time (5 years) at short notice can also be really important. There are alternative solutions that could be done from the other way around (Australian applying for a Korean visa) but none are as quick and easy, or as flexible, as the Resident Return visa. It's pretty much instant.

I wouldn't cast judgement over anyone in this position but definitely not without knowing their personal circumstances first. The idea of renouncing your birth citizenship is probably very easy and painless for those who will never be in a position to consider it.