Because bonds change value when interest rates change.

You buy a bond paying 3%, bond yields go up to 4% and you want to sell your bond, it will be less than the face value you bought it for. This isn't an issue if you want to keep it to maturity. But that's what SVB (and others) did and that creates its own problems.

If you're buying bonds, you're either parking money for a long time or you're betting on interest rates going down.