Because Miami has won a Super Bowl.
As for worst teams in the league, Bengals will probably be worse than the Bills and Lions. Unproven coach taking over a below average team. Washington probably won't win many games with a rookie QB. Giants were already shit last year before trading away most of their best players, so can't expect much there.
Lions should be good enough for 6-8 wins. They managed to whoop the Patriots last year.
Bills are good defensively, but have a horribly inaccurate QB; basically Ravens north. I would expect both to struggle to get to .500, but they're not doormats.