Our nation was instituted by oligarchs who quite naturally presumed themselves to be ordained by God and the natural order of things to dictate the terms and mechanisms of the new nation's government. And in so doing they wrote a Constitution that allowed for this presumption to fester.
When George Washington succeeded in winning our independence from Great Britain he was appointed our first president, but at that time, it was assumed that he would be our new "king", for life, by the men who set up this nation. And such an appointed "kingship" of a wealthy elite, by a wealthy elite, rather than heredity, is pretty much a definition of plutocracy. To George Washington's credit, he refused to accept such a kingship, and insisted that we elect new leaders periodically, but even then it was understood that these elected leaders would come from the ruling wealthy elite class of men. And it has been so ever since.
So the seeds were sown right from the beginning, and the doors of government were constructed to remain open to any would-be oligarchs right from the start. Our founders talked of freedom, justice, and opportunity for all, and I think they even believed it in their minds and hearts. But in the end they were essentially oligarchs, and they modeled their new nation's government after themselves. They never instilled any means of thwarting oligarchy because it never occurred to them that they should.