Because the machine is a physics simulation. Where users can set up simulated physical objects like barriers and bumpers, and you want the animation to display a simulation of what happens according to the laws of physics.
I think you may be missing the point if you think this is an animated rendering of a static setup?
The problem description is ”Could we make a really big tiled mechanism like the blue balls GIF? Where everyone contributes a small square?”.
There is a stated rationale for every other design choice, but the blue balls GIF is an animation, not a simulation. It didn't even start as a simulation. So why the choice of making this a simulation, not an application for users to animate tiles?
I think you may be missing the point if you think this is an animated rendering of a static setup?