Korean women realizes they can enjoy life, travel and stay young and beautiful longer being not married. Heck, even the men. It's the similar trend with millennials around the developed countries.
U get married, u get saddled with mortgage and kids, and inlaws. Your current income is enough for u to live a bachelor life, casual dating ( even living together) but marriage comes with a whole lot of other baggages.
Even if you don't plan to change partner, imagine if you and your wife rather spend the holidays in Thailand than visiting your in-laws and your savings on good foods and airticket than diapers and formula. How is that gonna sit with the relatives and the parents? Hence why would the couple wanna get married?