Why do ordinary (i.e. English, not American) people sing in American accents? Geordies, Brummies, carrot crunchers, cider drinkers, all of ’em. They all have almost indecipherable lilts to their voices when speaking, yet when they sing they magically attain an American accent.
The only exceptions I can think of is the lovely Cerys Matthews, boyo, and that geezer out of Blur.