Because its easier. And fun!
From a kid perspective
We wanted to both facilitate high-fidelity story creation and teach the concept of keyframe animation (which is initially very foreign). Add to that learning a professional-level animation program and you will have a confused and frustrated student - who just wants to make animations!
The mapping of our system is also highly intuitive. The kids (ages 9-11) all understood the relationship between the physical and digital figure within seconds.
From an interaction design perspective
It takes much less time to move limbs on a doll than locate each individual limb in an animation program, make another key frame, tween it, etc.
Though our program isn't as sophisticated as other animation tools, the user still feels agency over the input device (figure) without having the cumbersome task of tweening individual parts.