That's because Americans, unlike Europeans for example, aren't really bound by any pre-state history. There were no American tribes uniting, no real ethnicity, just fairly random people from around the world (though white Europeans mostly mattered). So the Constitution (and the Declaration of Independence, some other writings) is like a deal signed by those people with their government and they treat it like a deal -- it shouldn't be changed without a very good reason and consent from parties involved, the original intent matters, etc, etc.

The whole idea of a social contract makes much more sense there.