Our Declaration of Independence states that people "are endowed by their Creator with certain unalienable Rights."
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I believe you've answered your own question, if you think about it. Within the constitutional context, this is so because the document says this is so.
The Constitution says "this and that are true, signed, We the People." Voila - law of the land. This is one of the key assumptions upon which the framework of the Constitution rests, so as I said it's there because they decided to put it there.
Notice that it doesn't say "God," but "Creator." To you this may mean a big bearded guy in a white robe riding around on a cloud tossing thunderbolts at the naughty. To some fruitcake in Kalifornia it might be one of those traffic barriers they've been known to worship out there. To an atheist, it might mean evolution.
The point is, they are saying that man came into being with these innate freedoms already belonging to him, and the Constitution was created to preserve them because the framers knew that the business of all governments is to find ways to take those rights away, so they got "dibs" on them by establishing a Higher Authority which trumps mere government.
Boy, those founding fathers were really a bunch of thinkers, weren't they?