The main reason is the belief that low interest rates are supported by low inflation. Except inflation is not low! It was 2.5% at the last report. Thankfully, oil/gas prices are stable but the cost of just about everything else is increasing at a modest pace.

The 2002 - 2007 market rally cracked when the FED refused to lower rates in face of the credit bubble bursting. The reason for their reluctance was the annual inflation rate up to summer 2008 was near 5%! The Fed kept rates high and then the wall street banks started failing due to the collapse of credit and the dominos all fell down.

We are facing a similar risk in 2020-2021. If the supply chain out of Asia is disrupted, which appears to be the case, inflation is going to become a serious issue. What, then, does the Fed do?

Look at the 20% drop in the market in 2018 because rates were too tight. The idea that rates won't get tight again is going to be a great miscalculation by investors.

And note, my saying that rates will get right doesn't mean that the FED will be increasing the discount rate. It means that the ability of corporations to borrow at below market rates will be curtailed