Why are the main protagonists in the main stream anime, stupid?

[Claraviolet]

I don't get why to be honest.

Is it because they might attract attention? Or is it because they try to cover the overpowered character with some stupidity mixed, so that they won't be hated in the long run?

 

[Demon_Skeith]

They got to be something, and stupid seems easy to work with.

 

[NameGervin]

If there is a stupid main protagonist there's gotta be super genius side character.