I agree but there are some like Lilly Allen that don't.
Not all of them do...Check out Tom Felton. He's English and he sounds English when he sings.
Or maybe Americans sound British.
That's funny you should say that cause I thought the same thing and one day my british boyfriend commented that American singers sounded English haha.
It's because the way you're supposed to move your mouth when you sing eliminates a lot of accents. Even American's sound a bit different when they sing...well, if they're trained singers, anyway.