If Japan's 10-year yield is now at 3%, what still makes Japanese investors continue to buy US Treasury bonds ?

Japan's 10-year government bond yield reached 3% today. I know that Treasury still offers higher yields, however, I think Japanese investors are probably also considering the exchange rate and not just comparing two numbers.

I don't know if 3% is still not attractive enough, or if there are other reasons why US Treasury bonds are still worth holding at this level?