>>280870844
It's simple: They take inspiration from real life and from works outside of the anime/manga industry.
>One Piece
Mario, Zelda, comics, cartoons, movies, wrestling, science-fiction, gangster fiction, cowboy fiction, pirate fiction, premodern fiction, folklore, human history, etc
>Naruto, FMA, Bleach, Death Note, Soul Eater, etc
Final Fantasy and Zelda. And what made Naruto such a revolutionary character is that Kishi was the first to model his MC after Bart Simpson.