Because each side, Alt-Right and Progressive, are inherently authoritarian. Both hold to a particular utopia (which we ought to remember is derived from the Greek for “no-place”), wish everyone would act in such a way as to realise it, and are willing to coerce them using state power to do so.
The coming era of political violence
Brendan Markey-Towler

Which we have to thank Plato and his abuse of philosophy for personal and political gain for. Plato aimed for restoration of aristocratic governance of Athens with a leading role for his family.

He did not manage completely and tried to implement his system in several colonies. In the end, at the expense of opposing philosophical schools and systems that gained a bad reputation in the the following centuries, e.g. the socalled sophists, cynics, stoics, and even streaks of non-idealist naturalistic empiry. (Note most what we today ‘know’ about these schools we know through the characterization of their opponents.

One of the pupils of Plato was Aristotle (who remained more favorable to natural empiry and) managed to educate / motivate Alexander the Great to try to create a world-empire (that fostered exchange of ideas between East and West and) that then plummeted into the factional Diadoch wars of his generals.