This is thanks to decades of cartoons and movies where everyone playing a pirate was legally obligated to litter their speech with "arrs" and the like while assuming the intonation of a rowdy drunken Englishman. Unless your name is Johnny Depp, that is; then you're obligated to assume the intonation of Johnny Depp in every Johnny Depp movie ever.
"What do you mean? I'm doing a pirate voice now. This is my pirate voice. *mumble mumble*"
Granted, outside of the Pirates of the Caribbean franchise there haven't been a lot of pirate movies in the past few decades, but at the same time there's been no shortage of pirate or pirate-talking characters invading other genres: Like the Pirate Captain in The Venture Bros., the other Pirate Captain in The Simpsons, Steve the Pirate in Dodgeball and Agent Scurvy Pirateson in CSI: Miami.
Obviously, we know the "pirate accent" we hear in these shows and films is exaggerated, just like they would exaggerate a French or Mexican accent, but it must be based on something real, right?
The Venture Bros. wouldn't lie to us.