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Not everybody on a football team can be the quarterback. Not everybody on a football team can be a linebacker. You need linebackers and other defenses to protect the quarterback.
It's also like complaining that a car mechanic is not an electrician nor is like a carpenter. They're all a part of the same team and all have their role to play.
It's not like you have three people on the team doing everything.
The problem with that, though, is that the football team is almost always centered around the quarterback, who is clearly the most important piece of the puzzle. The quarterback is usually the one the gets the most glory when the team wins and most of the blame when the team loses. And, for professional teams, the quarterback is usually the highest paid player.
And similarly, a wizard in older editions of D&D is the quarterback of an adventuring party.