It is fairly easy to know where to begin a work. However, the ending comes after a long, hard journey to get there. During that time, the destination itself can change.
Ideas do not remain stagnant: they move like living things. What seemed to be the perfect ending can falter under scrutiny. The foundation is unsure, and thus it collapses.
Characters change throughout the course of a story: for the better or for the worse. This can have a tremendous impact on the outcome. And thus, it is best not determine the conclusion, until they have had their say.
There are determinations to be made: how should the ending be accomplished, who will affect the outcome, and what does it all mean? The entire work will be judged by the way it ended, so this cannot be arbitrary. All that came before must lead up to it.
A bad ending can ruin a story, while a good ending can save it. That which the audience sees last, they will remember first. To stubbornly ignore the wishes of the reader is to consign your work to a neglected shelf where the dust will collect.
But if you craft a satisfying ending, the story shall always be remembered for it. People will talk about the ending long after they have read the book. For the destination has changed: to one that is better for you and better for those who enjoy your work.