The 10-year Japanese Government Bond (JGB) yields 0.01%, while U.S. Treasurys similar issuance yields 0.72%. It seems like the investors like the JGB so much that they are willing to accept close-to-zero interests when lending to the Japanese Government. Both the U.S. Treasury Department and the Japanese Government borrow in the currency they print. Why is the case that only the U.S. Treasuries are considered as the risk-free assets?