As the underpinnings of the Left have repeatedly been shown to be false, those who became accolytes of the Left had to create new premises to shore up their faith. The premises have become increasingly distanced from reality.
A relatively small number of committed "progressives" in the media, academia, education and politics control most of the information flow, especially to Leftists. Thus, first the idea that Marxism would come from a revolution of the workers was discredited.
Then the idea that Marxism would be more productive than capitalism was discredited.
Then the idea that workers under Marxism would be better off than under capitalism was discredited.
Then the shift was that capitalism had a greater range between the wealthy and the poor was put forward as the reason to level all with Leftisim... except Leftist societies had an even greater gap between the Leftist rulers and the mass of poor workers. (Consider Venezuela, North Korea, Cuba).
So the Left's reason for being has devolved into Identity politics and critical race theory. It is irrational and unprovable. Those are its virtues.
Like Islam. The greatest good in Islam is spreading or strengthening the hold the ideology has on society. Even if you have to maim and murder people to do so.