And because that was long before the Great Vowel Shift, those giant pirates said, “aerrrrrrrgh!” instead of “arrrrrrgh!” (That was an inside joke for historical linguists.)
POSTSCRIPT: Robert Newton was the first to “talk like a pirate” in what has become a stereotypical way in his 1950’s portrayal of the famous Treasure Island character. The actor happened to have grown up in the area of England from where Robert Louis Stevenson’s fictional Long John Silver had come. “Arr” (rather than “argh”) was basically a synonym for “yes” in that area of England, and Newton decided to use that distinctive word and his native dialect in general. Because most pirates came from other areas of England and the world, it is unlikely that many pirates were prone to saying “arr” in routine conversation aboard ship.
For those too young to recall the popular movie character from the 1950’s (and the Wonderful World of Disney TV show of the 1960’s), here’s a illustrative compilation from an Australian sequel featuring Newton in his familiar role.