Common sense. Sanji is already a great cook and he's strong however, let's not forget he's also a strategist and good at stealth too.
Him being as strong as Luffy or even Zoro is completely nonsensical since they are both specialised in combat while Sanji is versatile. That's how Oda keeps them balanced. Zoro and Luffy as combatants are stronger but Sanji offers more to the crew other than combat. If he was as strong as Zoro or Luffy, he instantly becomes the most valuable crewmember and that is something that would never be the case