The Framers of the Constitution had few historical examples of successful governments that were not monarchies. They had the extremely short-lived direct democracy of ancient Athens (which wasn't very democratic, because only a small minority of the adult population could vote), the sad example of the Roman Republic (which was a very mixed system that makes ours look simple and easily comprehensible by comparison) that morphed into an autocracy in fact if not in theory, the rather short-lived Venetian Republic (which was even less democratic than the ancient Athenian democracy), and Switzerland, which is a republic in the form of a confederation. Oh, then there was the short-lived "Commonwealth"-republic of England under the dictatorship of Oliver Cromwell.
And their whole point in gaining independence was to fight against tyrannical government. They didn't want to trade royal tyranny for home-grown tyranny. They had tried the very decentralized Articles of Confederation, which proved not to work very well. They needed a central government that had more powers than the Confederation government had had, and they needed ways to keep the federal government from becoming tyrannical.
So gave the federal government more power--too much power in the eyes of the Anti-Federalists like George Mason and Patrick Henry--and they included all the things that we now call checks and balances, although those terms are not in the Constitution. Separating the federal structure into three branches, each of which can to some degree check actions of the other two (through vetoes, veto overrides, impeachment, ruling laws unconstitutional, etc.), is a check, or rather a series of checks. Making the Senate represent different interests than the House is another. Dividing powers between the states and the federal government is another.
And the Bill of Rights (1st 10 Amendments) put further restrictions on what the federal government could do. Unfortunately (IMO), since the ratification of the 10th Amendment, all amendments that have dealt with federal-state relationships have strengthened the federal government at the expense of the states and/or the people. The first three examples--the "Reconstruction Amendments"--were absolutely necessary, but nevertheless shifted the balance--13th Amendment abolishing slavery, 14th Amendment establishing birthright citizenship and equal protection of the laws, and 15th Amendment guaranteeing male-citizen voting rights. I would argue that subsequent amendments were less necessary. The 16th, income tax, was an ostensible attempt to level the economic playing field with a progressive tax. But it empowered the federal government to look into everyone's bank account. The 17th, direct election of U.S. Senators, as previously discussed, reduced the power of the states and weakened the check that the House and Senate are supposed to exercise upon each other. The 18th imposed national prohibition of alcohol, and the 19th guaranteed female suffrage in every state. Those four were the "Progressive-Era Amendments," and aside from their other effects for good or ill, they are consistent with Progressive efforts to strengthen the federal government and weaken the states.
To get back to my point, the Framers were attempting to create a government that would work, that would last, and that would not become tyrannical. Fairness was not the main goal, and democracy was something to be restrained rather than magnified.
Incidentally, Woodrow Wilson, the great Progressive POTUS (who was also the most racist POTUS since Andrew Johnson) was the first president to openly disdain the Constitution. He advocated a parliamentary system. He was lucky he didn't have one, as the Democrats lost control of Congress in the 1918 mid-terms, so he would have lost a vote of confidence right at the end of WWI and Henry Cabot Lodge, or some other Republican, would have been our chief negotiator at the peace conference in Paris.
I say that Wilson was the first POTUS to openly disdain the Constitution, but T. Roosevelt (earlier POTUS) pretty much disregarded it whenever it got in his way. He just didn't openly say that the Framers were idiots and that checks and balances were mischievous impediments to good government.
