Where else are rich people going to park their money? The demand for investable assets has gone way up because rich people have gotten much richer, and rich people around the world (such as newly minted emerging market billionaires) often prefer American assets for liquidity and good legal protections. I'm sure these investors would prefer that stocks earn more than 2.5% but that's just the going rate now. What alternatives do they have? Bonds yield basically nothing.