I don't know jack squat about the balancer or the bushings, but if I were on a gameshow and had to guess, I'd say that it's because internal combustion engines are inherently inconsistent and unstable, thus requiring a certain variability in the balancer?
If I somehow got this right on accident, I fully expect a participation trophy.
I will grant you one!
But I think you're right. The rotational speed of the engine varies quite a bit per rotation.
An engine only speeds up for 1/2 a rotation and spends 3/2s of a rotation slowing down.
Most of the slowdown happens on compression, just before it's 1/2 rotation of the power stroke.
Being a V-Twin, it's a little more steady than a single, but still lots of instantaneous changes in speed throughout each rotation.
The rubber provides some elasticity to this to smoothly reduce this rotational vibration.
Essentially, a harmonic balancer.
If you've ever taken apart the rear wheel assembly, there's a giant rubber piece inside that couples the rear drive to the wheel.
Part #92075 2005 Kawasaki VULCAN 750 (VN750-A21) Rear Wheel/Chain | Kawasaki Parts Warehouse
This reduces the rotational vibration/shock on the driveline. This also can help prolong tire life by not forcing harsh power pulses on it.