Now interest rates are much lower, but Koreans got used to paying $0 rent and anything above that feels like they are being ripped off.
One reason landlords like the system is because they can daisy-chain the purchase of multiple properties. They take the first cheonsae deposit and use it to purchase another property, then use the cheonsae deposit from the 2nd property to purchase a 3rd property, and so on...
Even if landlords don't receive monthly rent, they expect to earn money on the increased value of the property. Cheonsae contracts are normally 2 years, then they can ask for more deposit or sell the property for a profit.
More info: https://en.wikipedia.org/wiki/Jeonse