No collision detection. To do anything different requires considerable time to manually check for and correct each and every interaction within the body itself and with other objects and figures in the room. If word processors behaved like this all the letters and words you type would pile up in a little box on the screen. Then you'd need to drag each word out into the proper place on the document. Nonexistent representation of true human form.
Dance enlists the use of nearly every bone and muscle in the body.
We need a library of realistic body types, with typical and exceptional mobility ranges, correctly proportioned, and fully articulated. Dance animations need to be keyed to a particular beat in the music, not to the clock on the wall. The idea behind key frames is that you don't need to build and position every frame, just the important ones where the motion of a object changes. For example, you show the beginning and ending position of an arm swinging through an arc using two key frames. When it falls short you add more key frames as check points.