Don't be misled by the term amendment.
The Constitution was written and approved by the framers. When they were done they looked at the thing, and decided it's a good outline for running a country, now how does it apply to and effect the people? They then wrote the "Bill of Rights" to spell out the protections of the people from the government. They were written as amendments 1 through 10, so they would never be separated from the Constitution.... so the government couldn't just cast them aside. Only then was the package presented for the approval of the states, of the people.