Constitutions for the most part are not about describing human rights. They are about describing the method of governments - who is elected where and how, what authorities they have and so forth. There's usually some flowery preamble at the start about how awesome we all are in country -foo-, but the role of a constitution is generally to describe the structure of government.
I kind of agree with you but that is not really true. The American constitution clearly makes the case for fundamental rights such as Freedom of Expression, among others.
