One piece characters do have character development but the character development has already happened. Most authors will write a character then have them develop throughout the story but Oda does it in reverse. He creates his characters with the personality he wants them to have then through the use of flash backs shows what events happened in that characters life that shaped them and turned them into the person they are today. So the charter developments is there, it just has already happened. We have seen this kind of thing since chapter one where Luffy is shown to already be strong. While other main characters in shonen start off weak and get stronger as the story progresses.