"Well, first of all is the shape of it. It can’t be too big or too thick… Unless someone wants just that. Second, it must be waterproof, otherwise the motor will malfunction quickly. It also needs to be durable, to not get crushed by contractions.”
"So… Why not just make a somewhat hard plastic casing and the dip it in molten rubber for an outer layer or something?"
“Because the thicker the shell, the less and less of the vibrations can be felt through it. It’s a delicate balance, if you give it too small of a motor, you won’t feel anything. But you can’t give it one that is too big without making the whole thing bigger. And that translates to the spining weight too.”