Viscosity breakdown basically means when oil is old, it gets thinner, which means it has an easer time fitting thru gaps. So it can disappear (escape) quicker when it's old. Im no expert but my understanding is that it could also simply start vaporizing inside its compartment at normal operating temperature when its old, escaping by slowly losing its liquid form and escaping as a gas or mist.
The other thing about viscosity breakdown is that it means it's losing its ability to protect engine parts from wear, which is exactly the function it's supposed to perform.
So backing up what everyone else said, just from a different angle.