[throwaway290]

This has less to do with population density or prevalence of tourism and more with high-trust culture, tight communities, desire for predictable and safe immediate surroundings for one and one's family, fear of disruption or damage to property, disinclination to let vacationing strangers with unknown behavior and no responsibility or investment into well-being of the community live in your house (and next door to your neighbors, with whom you would presumably want to maintain good relations). Seoul is not in any way special here nor there is any "problem" to speak of--the situation strikes me as natural and how I would prefer it where I live, what about you?

[rafram]

No, the situation in Seoul is not “natural.” It’s not about tight communities or tourism, because those very high costs apply to Koreans renting long-term as well. Renting an apartment can cost tens of thousands of dollars in up-front deposits, and getting a mortgage (actually just a loan) to buy a house/apartment requires putting up a large asset that you already own as collateral.

In practice, both of those things mean that your parents need to be well off and willing to put tens of thousands of dollars and/or their house up as collateral for you to be able to find a place to live. It’s an insanely broken system that encourages economic stratification and keeps the vast majority of the population from ever hoping to own or even rent in the city.

[throwaway290]

I suppose even if you are a Korean and/or willing to sign a multi-year contract, owner's ability to have recourse is limited if you decide to break it early and vanish without a trace so high deposits are a more reliable measure to reduce the amount of incidental people.

Whether this has to do with high-trust society or just greed is another question, I like to think it's more of the former but it could be both...