Because he would have prevailed against Lee if he had had Lee’s manpower and resources, and Lee had had Grant’s?
Grant used what he had to his best advantage. He certainly didn’t have a massive advantage in manpower while fighting in the west. He also never made the same mistake twice - he learned. Lee repeated made the same mistakes. Further, Grant had a solid strategic vision while Lee had none. You can argue that Lee was a great tactician but Grant’s Vicksburg campaign was the greatest tactical campaign of the war.