Why does the leftist intelligentsia embrace delusional ideology & support symbolic but inneffective gestures?

Since the 1960's society has fallen victim to numerous failed and nonsensical ideologies from the left wing of politics. Examples include political correctness the failure of multiculturalism, relativism and many other post modern theories which petulantly reject generations of inherited wisdom for the sake of bratish change. This has undermined the fabric of society and contributed to moral decay, premissive liberalism, racial tension and more... how can academe and the inner urban elites get it so wrong and we perpetuate their failed ideas year after year?...