Because it's a Shonen- aka the bulk of it's fanbase are hormonally charged young boys. You'd think One Piece would buck that trend for those who actually read it but nope. Just a gigantic martial arts tournament I guess. /s

I'd be totally fine if Shanks turned out to be a conquerors haki diplomat who couldn't beat a central antagonist without his crew for shit. If anything that would be an interesting commentary on leadership and trust. But I guarantee if that happened half the fan base would feel betrayed because of Shonen obsession with hierarchy. It's really bizarre and honestly discounts the great storytelling of One Piece. This ain't DBZ kids.