It's not the case that some of us may not have the 800,000THB, It's more than the case that some of us don't want to park that amount of cash in a Thai Bank for 5 months.
Wise (Transferwise) now paying 4.17% interest on USD cash balances with them, so parking 800,000THB in USD for 5 months would cover the agent's fees and all the hassle.