Why does American animation have poor art, and a good story, and japenese vice versa?

American animated shows have good story lines, look at Batman Beyond, X-Men, Spider-Man, the upcoming Beowulf but poor animation, and japenese amimated shows have great animation like: Sailor Moon, Ghost in the Shell, Dragon Ball Z, Final Fantasy The Spirit Within, and Final Fantasy Advent Children, but poor story lines, that are weird, far fetched and make no sense.