Why do some owners ask for a one-time payment when buying a second-hand house through an intermediary?

If you make a mortgage, there is a process in advance. After the transfer formalities are completed in the housing exchange, the remaining house payment will be transferred from the bank to the owner. This process means that the house is already yours, but the owner has not actually received a large part of the house balance. This is just a concern and worry of the owner. Either the owner is not familiar with the safety and reliability of this process and has scruples; Or just worried that the bank will lend money for too long.