I think it is because the whole clutch surface doesnt end up touching the bell. Mostly only the middle part does so they put as little pad on there as is needed to engage the clutch. Probably also a cost saving method by the factory. I dont think it would hurt to line the whole clutch but I dont know that I would expect anything big to happen. Try replacing the spring with a little stiffer one.
Super cool tool btw. Make a bunch and sell them to treats! (edited)