Because they are part of the main characters. Simply that. Maybe I'm wrong and now they are not strong enough, but considering how fast character become stronger, Oda will always give them that push at the end.
If we're talking about, say, Crocodile vs Sanji, I might argue Sanji wins extreme diff due to his role as the main character as well as being one of the most valuable and powerful assets in an Emperor's crew, but against someone like Issho or Warcury.. Well, only so much I can do for the man. Some fights he just can't win no matter how much you try to use vague things like him being a main character or possibly having PoL.











