If the alternative is getting 0% yield on USD, why wouldn’t they trade the USD for a bond? And before you say “just buy EUR/gold/stocks” that just means someon

If the alternative is getting 0% yield on USD, why wouldn’t they trade the USD for a bond? And before you say “just buy EUR/gold/stocks” that just means someone else (the seller of EUR/stocks/gold) would now be in the above situation. Buying US Treasuries for USD has little to do with absolute risk of the bond, and everything to do with the non-existence difference in risk between the bond (US Treasury) and the currency (USD) it’s denominated in.