User blog comment:GrilloSama/Jinbe For 1st Mate/@comment-2A02:587:741F:3A00:E151:51B8:F255:E3A8-20170925140359/@comment-5254348-20170925193948

The same reason other people analyze the crew and come up with reasons why character will/won't become a crewmate. Plus, there have been plenty of times in the story where the characters have put emphasis on they're roles in the crew, even Luffy. Luffy may not care about the Strawhat Grand Fleet but he has put thought into his crew and their positions. They don't have a typical "hierarchical structure" but they do have structure.