Because every political system evolves over time and initial checks and balances envisaged by the architects of every such a system are no longer applicable or woefully obsolete.
The US political system has undergone a veritable social, judicial and political revolution (and one civil war) through the two and half centuries and greatly differs from that envisioned by the Founding Fathers, no matter how many conservatives use the phrase “what would the Founding Fathers say”.
Take for example, United Kingdom - Queen Elizabeth II operates in the same constitutional framework as Queen Victoria or the mad King George III you guys rebelled against, yet their respective positions inside the actual political landscape cannot be compared, despite the fact that the diminutive grand grandma is still the Head of Church of England and Prime Ministers kneel before her asking for permission to form the government.
That’s why many historians are concerned because there are - very possibly unfounded - parallels with the collapse of the Roman Republic in the 1st century BC.
What confused and disoriented contemporaries in Ancient Rome was that, outwardly nothing has changed. The Roman Senate held regular sessions, elections for the highest offices were regularly held in which ambitious politicians tirelessly campaigned for votes and courted public opinion.
Yet all those trappings, traditions and rituals became pointless as several rich, powerful men cut deals among themselves bypassing not only the established elites but the entire political framework by establishing a direct connection with the people, who loved them, among other things, for their showmanship.
And all those Roman knights and scions of old families couldn’t, to use that cliched term, think outside of the box - they couldn’t understand that all those offices and functions they aspired to and fought for in the political arena their whole adult lives suddenly became pointless and obsolete. They were blindsided when these new demagogues openly flaunted centuries-old basic laws of the Roman Republic - not bringing the legions inside the administrative borders of Rome proper (what Sulla did) or disobey a direct order from the Senate (Julius Caesar).
In Cicero’s letters to his one time protege Julius Caesar one can clearly feel the palpable sense of increasing dread as he implores Caesar to stop with the demagoguery once he defeated his rival Pompey the Great and return to regular political discourse and respect for ancient institutions. In other words, he’s asking for a pivot to the middle, which never came as there was no turning back.
Or in Cicero’s own words: “Those that are destroying the Republic are claiming to do so in order to protect it and restore the ancient traditions of our forefathers”