Because that’s the path they choose
Zoro has fist and feet and is capable of using them but never does in serious battles. That’s the difference between characters like Zoro and King
King doesn’t limit himself to just swordsmanship, marksmanship, or martial art. He fights freely doing whatever he wants
Zoro is capable of fighting outside of swordsmanship and can if it’s necessary but still sticks to just strengthening his swordsmanship
To the point Zoro complained that King wasn’t using his sword enough