They knew they couldn't so they made a Constitution that could be amended and be treated as a living document for those changes. They strove for a balance between individual rights and the duty of government; they knew it was imperfect, but counter that imperfection, they gave the future generations ways to make change to the Constitution and government as needed. And they gave that power to the people so that no one man or small group of men would hold sway over an entire nation for personal gain, profit and power.
What they didn't count on was that the people, at a certain point, turn weak and into a pack of cowards. Willing to give away everything to "feel safe" or to reclaim power that they feel other Americans shouldn't have. Out of fear and over a span of about 40 to 50 years, sold our nation to a pack of men who care only for their own personal profit and agenda, to the point where they pretty much just ignored the Constitution, who only had to say things like "See that gay man? He wants to make your kids gay so he can have sex with them!!" Or, "See that person with darker skin than your white skin...they more rights than you do, and they are going to have revenge on you!!"
They sold their lies and made people fear. Told people the grand lie that America was no longer great. Told them to vote for an incompetent whose only good point is that he's good at selling fear, mistrust and hate. And the promise they gave the people was.....WE WILL KEEP YOU SAFE AND RETURN YOU TO AN IMAGINARY WORLD THAT NEVER EXISTED EXCEPT IN YOUR MIND...and at the same time....WE WILL TAKE EVERYTHING AWAY FROM YOU AND YOU WILL ADORE AND THANK US FOR IT.
The Founders put faith in the people. And the people let them down. It was never the Founders' fault. It's our fault. It's the fault of people like me for not being able to stop it, and it was the fault of others for buying into it.