[Ultima]

Because Americans are silly? ^^;;
In Finding Nemo, only a few characters had an Australian accent, even though they were all supposed to live in/near Australia.
Other countries do this too. Characters in anime almost always speak perfect Japanese (or American English if it's the dub) even if they are supposed to be from elsewhere, and very rarely say anything in their 'native' tongue.

But anyways, ... I do think it has something to do with Americans not being able to understand other accents as well, and as soon as we hear an English accent, we go "Ooh! An accent! We're in another land!" I have noticed a few movies where I think "But doesn't this take place in _______? Why do they have American/English accents?" But it usually doesn't bother me much. I think casting has a lot to do with it too.

WOAH, rant. ... I think I'll just stop right here ^^;