Because Oda wants to hype everyone. In order to do so, the powerscaling has to be vague. Even if both Luffy and Kaidou want to be PK, placing them directly versus each other, Oda may not want Luffy surpassing Kaidou in individual strength to be accepted by everyone. Thats why Luffy requires help in taking down major players until now.
...Which is somewhat cowardly take for an shonen author imo. I don't mind it at all if Luffy needs help in taking down other villains like Katakuri or Cracker, its completely fine.
But needing the help in toppling down such a decisive PK-title candidate like Kaidou the WSC is so ridiculous and too timid for my taste. Luffy taking down Kaidou with help, just feels like if Zoro must take down Mihawk with help of Vista-King-Smoothie-Oden-Cracker-Pica lolol.
Even an excuse like thats Luffy's power, he turns enemies into allies, its so lame because its not like that is Luffy's only power. If readers having that statement as an anchor, then Luffy always needs babysitters to win. But isnt PK the one with the most freedom? If we take that statement as an anchor, its Luffy who needs to be the strongest ever to protect himself and his friends, the ultimate nakama babysitter. Which is why imo any of Luffy's fight against PK candidate, especially a WSC as PK candidate, shouldn't involve help in inflicting major damage to the enemy, and instead should show Luffy's powerlevel and willpower difference compared to any other fighter in his side.