As a wiseman once said, "Because if you face the goddamn facts...." (yeah, that's what he said). Anyway, anime really doesn't have it too good in terms of fluid animation. It's focused on detail, and otherwise uses keyframes to keep the costs down and save time, which means we usually have "static characters, camera panning, only mouths animated if that." It's the nature of the beast when the average cost of a show is about $120,000 an episode.
But once in awhile, you get some anime scenes that could be considered classic Disney quality: everything's animated. There is no limited animation to speak of. They're really trying. Their mouths aren't just flapping; they're synced to the words. Some anime series keep a high quality in terms of animation, even if does occasionally use these shortcuts (FLCL comes to mind).
That being said, what's the best animation you've ever seen in an anime?