Because they're tonal. Because of this new point of view (I thought old) that this is possible, traditionally. Just because a tune starts in one key, doesn't mean it will end that way. Here comes another can of worms. Em or Emaj is the same thing, sort of, in American music. (you can play an Em or maj scale as a legit ending.) Not so in traditional logic. In American POV, maj and min ends the same. American music is unique. Tradition does not think this way.