Why do the studios let this happen?!?!
I don't know that it's "the studios'" fault; there are so many niche companies involved in producing a film today that there's likely some question a the end of the day as to who exactly owns what. Add to that the sheer volume of material, storage requirements, no clear-cut means of "disposal" (that is, do they just put it up on CraigsList? Make a deal with the Devil -- like the Prop Store -- ? "Loan" it to Planet Hollywood ... oops, not an option anymore), and you've got something of conundrum.
Some of this thread reminds me of an incident with a former employer: I had become something of the
de facto historian so other employees (including senior management) would bring me prototypes that had been used to obtain patents, product used and returned by NASA, last known examples, and more.
I was away from the office for a week and came back to find all of it gone. "What happened?," I asked my boss. "Threw it all out," he told me. "Why?!" "I don't like to have old things around, I only want to see new stuff."
Those who forget history are doomed to repeat it.
But that's another story.