There are a few possible reasons why Green may have decided not to include a post-credits scene. One possibility is that he simply didn't feel the need for one. The film's ending is already quite conclusive, and there aren't any major loose ends that need to be tied up.
The absence of a post-credits scene, in my opinion, is a plus. It demonstrates Green's desire to deliver a comprehensive and fulfilling story rather than set up future features. It also permits the audience to leave the theater with a sense of closure rather than wondering what will happen next.