Because that's life. All this talk in the thread about the point of the movie being "women are the devil brehs" it's so short sighted, and no diss to anyone when I say that, so please don't take it that way. The big thing of the movie is the nature of human beings and the lies we tell and masks we wear and like many Fincher movies, life doesn't always have a "happy" ending. It has an ending that the characters involved are satisfied with but are they really happy? Nick and Amy both decided in the end to play these roles of Nick and Amy, the happy perfect couple because it was easier than having to be real and having to be who they really are. There are maybe 2 or 3 good decent people in the whole flick and Nick and Amy aren't in that group; they both manipulate to get what they want, they lie, they cheat, and know that who they pretend to be is a lot better than who they really are.
