Thick Foam Rear Storage Boxes

Why do the storage boxes under the rear deck have 1.75 inch thick foam walls rather than a thinner plastic, like over the 12V battery? Seems like this thick foam wastes a lot of storage space, and may break down over time. My 2003 Vibe (same as Toyota Matrix) used nice thin, durable plastic & also on top of the spare tire. Are there any 3rd party replacement boxes available?

 

We are not suggesting removing the under rear deck storage boxes, but making reducing their 1.5+ inch thick walls that wastes storage space. Note that the lightweight folding plastic-clad?? rear deck material is 0.75 inch thick, which should also be strong enough for the storage boxes, sound deadening and directing air to traction & 12V batteries.

We like the storage boxes, but want more usable space in them!