One of the great misconceptions of the pre-nuptial agreement is that, as a legally binding contract, once everything has been signed "on the dotted line," this means that the document can no longer be questioned or contested. In fact, nothing could be further from the truth. While a pre-nup is indeed designed to lock down certain things, and a provide some structure and certainty in the event of a divorce, that doesn't mean that it can't be contested.
One of the things that many divorce lawyers will tell you is the importance of having a prenuptial agreement in place and signed by both partners. We'd love to believe that a marriage, once begun, really will last "till death do us part," but statistically, it's already a well-known fact that not every marriage comes to an end this way, and a significant number are resolved through a divorce.
There is a belief that a prenuptial or premarital agreement is in some sense asking a marriage to fail, or else it shows that the two people marrying are sure it won't last. After all, why else would you sign a contract before marrying that will only take effect if you get divorced? If everything goes well and the married couple sticks together, there's no need for a prenuptial.