Because those investors are betting on “the most realistic outcome” and not selling doom and gloom, or trying to hamper Trump’s re-election.
The answer is that people who understand the causes of past recessions see this induced recession as different. The normal reason for an economic contraction are because the Federal Reserve tightens credit and the money supply which reduces liquidity. Businesses can’t easily borrow to expand and they lay people off to reduce operating costs.
The Great Depression was caused by the failure of banks who had invested in the Stock Market on margin and were unable to meet their margin calls. The Fed stupidly tightened credit and reserves to prevent the banks from selling their gold reserves to remain solvent and the economy was strangled for years.
The financial crisis in 2008 was another failure of banks that caused businesses to pull back on borrowing and reduce expansion plans.
The unemployment of today was induced by the stay-at-home orders that had nothing to do with market liquidity. The Fed has gone all in on maintaining liquidity to solvent businesses who should recover quickly once people are able to get back to normal and forget their fear. Investors know the old rule “The Fed is your friend”.