Comment by babypuncher

The Japanese way is faster and cheaper, which is important when you're on a tight schedule. Simple mouth animations are easier to draw. They also have the benefit of looking less jarring when dubbed multiple languages, which is important as a lot of Anime is just as popular outside Japan as within.

There is a middle ground, seen in 2D western animation made on a lower budget than your typical Disney fare. Artists will create a mouth chart which includes drawings for a wide range of English phonemes. They are then used as a guide for drawing. There are software tools that assist in this process, making it a lot easier to do in a short amount of time. This is probably how South Park manages to do real accurate lip syncing while working on a 6 day production cycle.