As other people have mentioned, its about not being able to afford to buy a house to raise kids in.
I do want to also mention though, a unique Korean system called "Jeonse", which is a large deposit that is 50-80% of the property value. After putting in the deposit, you can live there on yearly basis without paying rent. This system has been and is being used as property ladder for young families (as they don't have to "lose" money on rent) but as "Jeonse" is pegged to property value, they've been rapidly rising as well AND frauds around Jeonse has been on rise, in which case you can lose the whole deposit or be tied up in court battle for years.