Well, if it is there then it is simply there because it is in common usage in North America. It is only in my copy of the Cambridge American English Dictionary, not the British version. It does have origins going back to the 4th century in these islands. However, it fell into disuse and appears to have been picked up again in the age of Websters' America.
I stand corrected on there being no such word, because of course the word does exist. However, the use of 'gotten' is regarded as very poor English on both sides of the Atlantic and especially so here because it essentailly fell out of common use and it appears very strange when used in the UK. Certainly an English exam board would not like the word used and I'd certainly never allow a letter out of my office with that word contained on the page.
Thus speaks someone who is dis dyslexx deslisx .... I can't spell lol
It is simply listed as an American word in the Cambridge Dictonary, it is simply not a British English word. It has always been in Websters of course.