Because their constituency was first built of unwashed workers and peasants that tend to multiply, but not prosper. The old Left had to speak their language. But then sometime in the 90s the great cataclysmic shift occurred; the managerial elite moved the production of material goods to China and other workforce-rich fields. Having a baby became a luxury, not a social duty.
The Left had to re-purpose their demagogy to suit the new electorate. And they chose the poorly educated spoiled youth with raging hormones, women, and servants envious of their masters fighting the same "service economy" pie, where the masters get rich while the servants get into debt to keep up the appearances.
It is well-known fact among primatologists, that young female chimps if they don't have young babies of their own, tend to care for other babies, massively "spoiling" them with attention and if there are no babies around, they grab teddy bear or even a chunk of wood of the baby size and play "care" games around it. Childless and downtrodden young male chimps, on other hand, tend to be utterly disinterested in babies, and they get their hands on teddy bear, they tear it into pieces.
And if there is nothing of the kind, chimps get rebellious and hysterical, which is dangerous for the leadership.
Leading such an electorate for the New Left requires this mixture of care for spoiled brats, welfare for childless females in the shape of teddy bears substituting real children, and welfare for useless males in the shape of virtual toys safe to destroy, all while balancing on a precipice of mass hysteria and rebellion. Inventions of new scary monsters (climate change, COVID) helps to stabilize the New Left with their electorate as it diverts their attention to something unfathomable and resembling ancient, angry, and unpredictable gods that we ought to worship to keep them from destroying us all while the leftist leaders assume the roles of priests.
Fighting dirty with the new Left would be dangerous for the whole primate population.
The real recipe for stabilization, in my humble opinion, would be to rebuild national economies to create independent and self-sufficient primate populations; they will calm down naturally, dealing with real matters of life and death.
https://www.spectator.co.uk/article/shouldn-t-we-feel-sorry-for-hunter-biden-