I lot of fiction, regardless of culture or target demographic, is incredibly protagonist-centric. The main character is often the thing that holds the setting, premise, concept, and the entirety of the remaining cast together.

Think of any number of fictional series you've watched. Then think about what would happen to that show's 'world' if the main character said "You know what? ...Nah. I'll pass." Odds are that the whole thing falls apart. Nobody else around has the charm, gumption, knowledge, skill, and/or innate talent to make the series compelling. All the supporting characters existed primarily to push and guide the protagonist in the direction he needed. Every location, set-piece, trial, and tribulation helped him discover something about himself and become more suited to his people-saving task. In many cases, the lack of that one character makes everything else completely fall apart: Without the protagonist rising to the challenge, literally nobody else in the story is capable of doing it. If they were, they'd be the protagonist.

Even objectively good or otherwise well-written series can sometimes fall into this pitfall. Even the flimsiest of motives (death or near-death of a loved one being the most common) can sometimes excuse a lack of main character's depth if the story he's caught up in is interesting enough. As soon as the protagonist in this type of story stops trying to 'protect everyone' then the series grinds to a halt and the villain wins, because the show was never set up in a way that allows for another character to step up to the task. Most shonen battle series are essentially looping "Hero's Journeys" and once you remove the hero, nothing is left.