Summary: Humans are still considered too primitive to join the Galactic Alliance.
One day the people of earth will take their place in the Galactic Alliance, and fall under the jurisdiction of the Shadow Proclamation, but that is still far in the future. At this point in earth’s history, before mankind makes the acquaintance of the many races who make up the Alliance, they’re sort of on their own, which is why they need him so badly.
Earth is a proscribed planet, off limits to most races, but that doesn’t keep others from trying to invade.
If humans are to survive long enough to join the alliance, someone needs to protect them.