A common argument you see in the modern age is that the writers of the U.S. Constitution did not intend for it to remain static and never change. They knew that the people of America would change, as would society and the culture. So they allowed for an amendment to be made by either the House and Senate doing a two-thirds majority vote, or two thirds of the state legislatures. It is through this process of amending that the foundational component of the U.S. government can change as times changes.
The U.S. Constitution was written in a time where the states were fractured, and the Articles of Confederation were doing a poor job of keeping the country together. That's why they framers of the U.S. Constitution created a federalist and more centralized government. A “more perfect union” refers to creating a better government with all of the states interconnected. That couldn't happen without a process of the states being able to affect things at the federal level, with a balance of powers from the federal to state level. Keep in mind that 2/3 of the state legislatures can amend the constitution, as well as Congressman and Senators from these states being able to amend the U.S. Constitution in 2/3 in both the House and the Senate.