Isn’t Jinbe actually stronger than Sanji and that makes Sanji the fourth strongest member of the Straw Hats? If so, that would mean the gap between Zoro and Sanji might be bigger than we think.
I think that most of them don’t use Haki for similar reasons as to why none of them are logia users, or why Luffy is just a paramecia and Zoro and Sanji don’t use devil fruit powers. It’s likely because Oda wants them to be underdogs and wants to write more creative fights for the Straw Hat Pirates.