Because it doesn't make much sense either?
Yes, something horrible happened on that island, so horrible that Garp and Roger had to team up to stop him from pursuing his goals. Yes, the Hero of the Marines and the future Pirate King had to combine forces against him! No, I can't tell you who he is--we have erased any mention of him from history books. In fact, we have erased that island, too. In fact in fact, nobody would even know about Garp and Roger if I were yapping about it right now--whoopsies!
Like, why even make up a cover-up story for something that shouldn't be known in the first place, and a cover-up that raises more questions than it answers at that? It's sloppy work.
