If you read the actual Halo lore outside the games, the Human government prior to the war, for hundreds of years mind you, has been a fascist junta police state that basically exists only to serve the army and navy. During the original Human civil war led by the People’s Front (Communists based on Reach), all communists were exterminated, and the Spartan project, superhuman and mentally broken child soldiers, were designed to hunt down and eradicate any leftist resistance movement that still exists. You even start Halo: Reach on a mission to go hunt down some rebels, as a bunch of communications relays get blown and thats blamed on the communists, as it wasn’t known that the Covenant had already infiltrated Reach.

Hell, during the war the UNSC essentially coups the few vestiges of “democratic” control, and then refuses to give up their control after the war, even after they “return” control back to the civilian government.