What do they have? Yield.

Take a look at the 30 day yield for VOO versus BND. Keep in mind that BND is stuck with previous interest rates, but are now buying the new interest rates.

When interest rates go up, Yield goes up. When Yield is up, the price for old bonds goes down, because nobody want to buy your bond when they can get a new one with a better yield.

Meanwhile, the opposite is also true. When the interest rate drops, your old bond has the old yield and the price inflates.

You are only looking at the bond price. How many years ago did a stick market crash occur? And how many years ago were their high interest rates?

The government has been propping up the market by keeping interest rates at zero.