So why then are US public sector pensions (which own a ton of the above assets) a looming disaster that could trigger the next great financial crisis?

Several reasons, ranging from negligence and criminality.

Let's start with the fact that Wall Street preys on the ignorance of pension fund managers to extract huge fees for little or no excess return. Here's a video in which pension expert and "forensic lawyer" Ted Siedle lays it all out for Peak Prosperity's Chris Martenson: