Why is the riba of Nasi'ah not applicable to currencies as it is to gold and silver?

Lending and borrowing gold and silver are permissible in Islamic law, whether they are dinars, jewelry, or ingots. A person is allowed to borrow gold with the intention of returning an equivalent amount later, and this is by consensus of the scholars. The Islamic prohibition applies specifically to the deferred or unequal exchange of gold and silver with each other, which differs from a benevolent loan (Qard Hasan). Riba an-Nasi'ah (interest due to delay) and Riba al-Fadl (interest due to excess) apply to monetary currencies, gold, and silver. Paper currencies have the same rulings as gold and silver concerning usury (riba) and zakat.