U.S. interest rates might be low, but they're a lot higher than interest rates in most of the developed world.
The big picture: The benchmark 10-year bond yield is negative in Germany, the Netherlands, Switzerland and Japan; it's also this close to going negative in France. Even Greece has seen its 10-year bond yield fall to just 2.4%, an all-time low.
Be smart: You'll see a lot of chatter about how the investors in these bonds would get a better return were they to just stash cash under a mattress. Ignore that chatter. If you're an institutional investor managing trillions of dollars in assets, you can't convert that money into cash, and while a bank will probably pay you 0% for that money, you still end up taking significant counterparty risk. Bonds have negative yields for good reason.