A big quirk of theatre is that we generally don't have nearly enough $ to do what we want - but we spend a fair amount each show (depending of course on where you are) to then turn around and throw it out. While some places do use a larger amount of stock, or are able to store larger amounts of previous built pieces, unfortunately money still enters into the equation. Storage doesn't come cheaply, plus you have time expenditures in terms or storing, organizing, maintaining, and transporting saved pieces. And that sometimes makes it cheaper to rebuild a piece instead of keeping it - plus allowing the designer customization of said unit.
What that means to the environment is that we buy lots of stuff, add chemicals and then trash it. The site listed below discussed alternatives that are more environmentally safe.