The Constitution is the source code of our country. It
literally describes what rights we have (edit: it is not an exhaustive list to be clear), thus why we have a document called “the Bill of Rights” which is a set of constitutional amendments that enumerate a set of rights that the government does not have the power to infringe.
So, respectfully, I actually see your confusion as a non-American; most countries don’t treat the concept of a constitution with any seriousness.
> but I don't really see "because it was decided so a long time ago" as particularly good reason to ordain something as a sacred right.
That is, again, literally how it works. For example, that is why here in the United States it is not illegal to deny that the holocaust happened, whereas in Germany or France you will get fined and thrown in jail.
Specifically Supreme Court decisions rely on precedent from previous cases; this is called Stare Decisis. Precedent is only to be overturned when clearly the wrong call was made (for example the SCOTUS decision permitting Japanese internment camps)
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To summarize, the genesis of America lies in the notion that we have inalienable rights that are not granted by the government but rather given by God (I think of this metaphorically since I don’t believe in a god). Thus the constitution just makes it explicit to the government that we have these rights and always did; as opposed to the government graciously extending these rights.
So in conclusion, despite the implication of your “Quran” question...yes, it really is a sacred document. Not just bevause it is the source code for our country, but because the ideas contsined within it are revolutionary and beautiful. For evidence of how revolutionary they are, I submit your (good-faith) incredulity with the entire concept :P