Because seeing a character actually have an internal struggle is very entertaining and good for character growth. Like Ichigo coming to terms with his inner hollow and Zangetsu
What character growth? What is there too grow for him? Sanji had a whole arc about him, how he is the person he is, why he is more kind than anybody else, all the struggle he had to deal with because of his personality, why his Mother died for that, etc.
Yet people out here want to see Sanji turn into one of his brothers...like wtf?