Why do we have to amend the constitution?

Answer 1

Because in time the majority may come to find its provisions inadequate. It was intended as a living document something to be changed from time to time to better provide necessary freedoms or in order to benefit the common good.

Answer 2

Because of moral progress, and because the powers of the federal government are limited by the constitution; therefore, sometimes an unfair situation can only be addressed by modifying the constitution.

The original constitution and Bill of Rights, for example, didn't give women the right to vote. Over time, more and more people came to agree that women should be allowed to vote. However, the federal government couldn't do this by simply passing a law, because the Constitution established the rules for voting eligibility, and the federal government didn't have the authority to instruct all the states to change their rules. A constitutional amendment is binding on all the states (even, once ratified, on the ones who didn't agree to ratify it).

In addition, the constitution is a limitation on the federal government as to what laws it may pass. It's a guarantee that, no matter how the winds of politics may blow, the Congress may not simply pass a law to undo this change and (for instance) strip women of the right to vote. Once we're really sure that something is right, we put it in the Constitution to make it harder to go back to the wrong way in the future.