Because he trains every freaking second he is awake?
backstory: he was fighting and training before even getting a master, then he got a master and trained even more, lost Kuina, trained even more until he was an adult, keeps training on the freaking ship whenever they aren't doing anything, then gets trained by the World Strongest Swordsman, still keeps on fucking training, gets in Wano, gets a new powerup in Enma that also needs to be trained or otherwise you die
he just doesn't stop training and he got the best master he could get in the world for his goals
backstory: he was fighting and training before even getting a master, then he got a master and trained even more, lost Kuina, trained even more until he was an adult, keeps training on the freaking ship whenever they aren't doing anything, then gets trained by the World Strongest Swordsman, still keeps on fucking training, gets in Wano, gets a new powerup in Enma that also needs to be trained or otherwise you die
he just doesn't stop training and he got the best master he could get in the world for his goals