I mean, the main character, Westley, is literally The Dread Pirate Roberts. And there's swashbuckling, sailing, and a ragtag group of outlaws who come together despite all odds to fight the crown and take it's treasure (Buttercup).
I'm not taking away from your very valid points, but I feel like a pirate adventure has to spend a majority of its time at sea on ships and The Princess Bride does not.
It does, however, make an excellent Land Pirate movie.