Why is Zakah obligatory on a deposit (*wadi'ah*) immediately upon ownership, unlike other debts?

Primary text

A deposit is treated as if it is in the owner's possession because the custodian (*mustawdi'*) acts as the owner's representative for its safekeeping, making his custody equivalent to the owner's custody. Zakah is due for the preceding period because it is owned property from which benefit can be derived, necessitating Zakah payment like all other possessions.