That is because legal tender is not in circulation at par with promises to pay credit.
The requirement of repayment of debt is against Public Policy since the legal tender was not loaned [nor in circulation] no one can demand payment in any [particular] form of coin or currency or legal tender and repayment [or payment] need only be made in equivalent kind such as a negotiable security instrument. How is credit or money created? VIA YOUR SIGNATURE as a BORROWER. Think about it..