It’s not in all anime, it’s primarily used in the shonen demographic. It’s overused because the primary demographic is children they’re trying to teach kids that relying on friends is cool, especially since guys love doing everything by themselves. That’s why a lot of shonen anime (especially Shonen Jump) are pretty direct about a lot of stuff, because their target audience is young.
That’s also why dumb protagonists are common in shonen anime, because that’s the easiest way to have an audience surrogate to explain the world and such for the audience. If you make the character dumb, it gives the other characters a reason to explain everything.