Materials to reduce sidewall flex & vibration ?

Heavy paving stones have long been placed on top of subwoofer cabinets by owners, as have bags of sand or lead shot.

Please help me understand this. Is this a work around for a poor cabinet design-build? I can't imagine having to do something like this to my speakers.

Thanks guys.
Dentdog,Granite sounds ideal. For a business all I need is a warehouse and laser cutter for the granite; but, I'm retired so that won't work.
bdp,hard floor tiles,perhaps black, are interesting.
I mentioned fiberglass/carbon fiber as they are more rigid that rock by weight and I'm hoping someone here has experience with them, perhaps automotive or art related.

donjr, that's correct; and I would say at least 95% of cabinet sidewalls flex.
I don't really know if it will improve sound any; but it's idea I want to investigate.

donjr, that's correct; and I would say at least 95% of cabinet sidewalls flex.

Cabinet flex was/is more of a concern with old monkey coffins. Its pretty much non existent in todays narrower speakers with more emphasis placed on cabinet designs, thicker baffle and walls constructed from superior material, and internal bracing.    

Eliminating resonance by minimizing cabinet flex will improve sound, more or less. But I think your approach is all wrong. I go about it on the inside with hardwood dowel. Or by thickening wall density by laminating similar material to existing. Or some of both.