Because of plot. Why did Crocodile decide to nuke Alabasta only once the strawhats appeared? Why did Enel decide to nuke Skypiea only once the strawhats appeared? Why did Hody try to take over Fishman Island only once the strawhats appeared? Why did the WG only decide to execute Vegapunkt when the strawhats appeared?
Because nothing happens unless the protagonists are present. Blud's overcomplicating a shonen manga.
So I'm still waiting to know how that slave attacked a WG ship to steal the fruit the WG and Gorosei were hunting for centuries. If Shanks is associated with the WG, it's literally in a way where the Gorosei bow down to Shanks and suck him. Saint Shanks can literally fuck them however he wants to and they will still respect him, sit down and listen to when he speaks, while they call Akainu, the most loyal soldier, a worthless dog.