Exactly right. Also you need to think about the sheer amount of dollars that China and Japan have. There are only so many investments that can give you a return

Exactly right. Also you need to think about the sheer amount of dollars that China and Japan have. There are only so many investments that can give you a return on 1 trillion dollars without significantly adding to the risk of losing the principal. Treasury bills are essentially no more risky than the cash that you trade for them.