DIY Easy to Build HEAVY speaker stands

Perhaps you mean "incredibly rigid", not "incredibly ridged" - I can't see any ridges at all!?
Actually PVC is a lot less rigid than any structural metal. But it is very well damped which is what matters here - being rigid can mean very little damping so its not actually desirable (other than to be solid enough to stay put).

Youngs modulus hard PVC: 3.3 GPa
Youngs modulus aluminium: 70 GPa
Steel: 210 GPa