How does smart contracts enable automation that centralized institutions can’t do?

How does smart contracts automate processes like the clearing house in finance or reduce the cost of delays between buyers and sellers that the traditional non-blockchain automation can't do? I understand that the smart contracts on a blockchain can reduce the "cost of trust" by making the transactions more transparent, but the automation can be done in a centralized institutions as well.

Could someone explain it in layman's terms?

Thank you.