Um, amendments are in fact in the constitution. That's what an amendment is - a change from the original text.
I'm sure you could get pedantic on this matter, but the subject of OP was related to structure and function of the US government. This is the purview of the constitution and not the first 10 amendments.
Functionally, they may serve a different purpose. So does the preamble, but it's still a part of the constitution. As is the bill of rights.