I think this is the right place to look.
At the bare minimum, back then the founders wrote a constitution designed AND INTENDED to be amended, if not rewritten every once in a while.
As of like 80 years ago people started to get it in their head that the USA was somehow the greatest nation to ever exist and could never be better. Maybe because of how it helped out in WWII? Maybe because of how the economy boomed following the war? Who knows. But Nationalism has certainly put the nails in the coffin.