Tenants like to rent under jeonse because it feels like free rent. The reality is that many tenants need to take out loans to make the jeonse deposit amount. Landlords make more with monthly rent because $2k of monthly rent is a higher return than you can make than receiving a $300k lump some to invest over the two year contract.
The reason why landlords rent under jeonse usually becomes they are really betting on property price appreciation. For the landlord to buy a $500k apartment, they only need to come up with $200k. They can use the $300k jeonse for the rest. When real estate prices increase, things go well. The apartment price appreciates to $800k and 2 years later, they can increase the jeonse price from $300k to $500k. Now you have $200k in extra cash to invest in your next property.
The opposite happens when real estate prices go down as they have been for the past 6-12 months. A landlord rented their property for $300k jeonse but now that prices have depreciated, they can only receive $200k on the market. When the current tenant's lease expires, they are supposed to get the $300k back but the landlord doesn't have this amount available in cash. They need to find a new renter and get money from them first.
With low interest rates, people could afford high jeonse amounts. These high jeonse amounts would lead to properties appreciating even more which would lead to even higher jeonse amounts. Now, interest rates are rising and you see cases like these.