This glib observation conceals a rather deep truth.
Difference between Nazi and Communist is when you say how horrible Nazis have been, they don't say, "Well, real Nazism has never been tried"
— Frank J. Fleming (@IMAO_) August 4, 2017
The Left are political rationalists, and the Right are not. Michael Oakeshott advanced the best and most withering critique of political rationalism. FEE.org has a good summary of his thinking.
So what is the substance of Oakeshott’s critique of rationalism? As he saw it, the primary feature of the rationalist approach is the belief that the essentials of any human practice can be conveyed adequately by means of a “guidebook” comprising explicitly stated rules, formalized technical procedures, and general abstract principles. Such a belief implies that understanding a theoretical model for some subject is all that is required for its mastery. Indeed, to attend to other features of a practice, such as experienced participants’ rules of thumb or tacit understandings on how to proceed in the domain, could serve only to impede the necessary rational reconstruction of the subject in question.
To the contrary, Oakeshott argues that the rationalist, in awarding theory primacy over practice, has gotten things exactly backwards: The theoretical understanding of some activity is always the child of practical know-how, and never its parent. In fact, he sees the dependence of theory on practice as being so unavoidable that not only is the rationalist incapable of skillful performances guided solely by theory, he is not even able to stick to his purported guidelines while performing poorly. Instead he inevitably will fall back on some tradition of how to proceed in order to give context to his abstract instructions. (This is similar to Wittgenstein’s insight that every attempt to follow a set of formalized rules necessarily is grounded on informal customs and practices that determine what it means to follow a rule “correctly”—the formal rules cannot also embody their own, “correct” inter-pretation because any effort to incorporate that interpretation into the first-level rules would create a set of “meta-rules” themselves requiring meta-meta-rules to guide the interpretation of the meta-rules, and so on, in an infinite regress.)
The Left will never learn from experience because they actually believe that experience is derived from ideas rather than ideas from experience.
What looks like a worldwide political debate is actually a clash, within Western civilization, over two irreconcilable philosophies: empiricism and rationalism.