because the phenomena Keynes described couldn’t happen in an economy of perfectly rational individuals and perfectly functioning markets.
Then came the 2008 crisis and its aftermath, which demonstrated that Keynes had been right all along.
The slump reflected a collapse in demand; governments that responded with deficit spending were able to mitigate the downturn, while those that practiced fiscal austerity made it worse. And the anti-Keynesian theories that had dominated the journals for several decades proved perfectly useless.
Many economists entered the crisis ignorant of basic concepts that had been worked out many decades earlier, because you couldn’t publish those concepts in the journals or teach them in many (not all) graduate programs.
Paul Krugman NYT 1 June 2021
Keynes lived an extraordinary and vivid life
Why do economists continue to get it so wrong? Lucas