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@Emanatronic 2 Words: Rotation shot.

@Project No words:

Rapture probably took thousands of hours to model and composite before anything could even happen. Draw that thing up in a day and you're ready to animate your first scene. Draw up another scene and you animate the second one.

If you're lip-syncing, you can get away with leaving your entire character except for its mouth completely static. No such thing in three-dimensional animation. Just draw up a character in two hours and start lip-syncing.  

@Emanatronic You realise that with 3 animation you just gotta make the assets and you're pretty much done. All you do is go from pose to pose and you let the program do the rest. With 2D animation you do this yes but it's purely for movement as you can't animate each limb of a character unless it's a puppet. Even so it only works with one angle. with 3D animation you can literally do the whole animation and add camera angles as an after thought.

As you can see with this scene the only with a tween is each building and the actual movement of the character, everything else is hand drawn. Not to mention with each time the lighting changes, the animators have to animate each individual thing that casts a shadow. 

Also the whole point of 2D animation is to make something 2D look 3D.