Because as interest rates go up investors lose money. The safest investment on Earth doesn't mean you can't lose money, pretty basic stuff actually.
You think because you invest in US Govt bonds you always make money? You always get a return, but the price of the underlying can go up as well as down. You need to do a bit of reading.
A bit like gold, you know the old saying, "as good as gold"? it's not so good at the moment.
