One Piece Forum

Argument 1: It can be assumed that if they were worthy of being classed in the same group as Luffy, then they are at least as strong. And for the two years that the Straw Hats were training, none of them have found One Piece. Thus, it's safe to assume that they were either killed soon after beginning their journey, due to not training for it, or have become much stronger and have merely not reached it yet. Or perhaps Blackbeard has been picking them off, one by one. His new desire for Devil Fruit powers would undoubtedly be satisfied on turning his body into a fortress, complete with armories, controlling ages, magnetism, voodoo, dinosaur, and... rooms. Either way, they have a headstart. It can be assumed that provided they're not all dead, and the distance or difficulty is just that much, then it stands to reason that it'll take another two years, minimally, for the Straw Hats to catch up.

Argument 2: Shanks has no desire to be the Pirate King. He's long since placed all his bets on Luffy. Even if he knows where Raftel, and One Piece, is, he won't go after it. If he did, and found it, he'd be Pirate King. Even if he turned it down and left it, people would still call him Pirate King, and anyone who found it afterwards prior to his death would mean nothing. Thus, he has no ulterior motives. He bet his life, his arm, and his hat on Luffy becoming the King of the Pirates. He's all in, and not about to do anything to risk a loss.