Part of the answer may be a liquid hidden inside the plastic.
PVC is naturally quite rigid. To make flexible cable insulation, flooring, roofing membranes, and many other products, manufacturers add molecules called plasticizers.
But making PVC soft on Day 1 isn't enough.
Imagine an electrical cable installed inside a building. It may remain there for decades. Heat comes and goes. Seasons change. Electricity warms the cable. If its plasticizer gradually evaporates or escapes, the insulation can become harder and less flexible.
So chemists learned to design plasticizers for endurance.
Global Chemical Database