The reason for this is that rich people and investment funds have more money than they can think of good ways to invest. Alex Tabarrok:
The global savings glut which drives asset prices higher and makes them more volatile is very much still with us. Around the world there is now over 15 trillion in negative interest debt.Think about that: $15 trillion for which nobody can think of a good use, just parked in accounts where it shrinks by the day.
I cannot think of any solution other than to run up the taxes on the rich and spend the money on things we need. Otherwise it will just keep sloshing around the world, distorting the economy and creating crises.