Where the heck do you find unopened copies of these things?
Occasionally, for example, on ebay sometimes. However, the prices called up there are usually correspondingly extreme. So I wouldn't buy that either. And probably just as some have old and unpainted miniatures at home.
In individual cases also in my closet. For example, I still have an OVP Space Hulk (third generation) from 2009. I bought that at the time, but for professional and time reasons I never got around to dealing with it. In between you forget it for a while or other interests are and become more important. Or you buy something in advance and need less, or your plans change.
It will and should probably be similar with other hobbyists who still have old boxes or untouched sprues or miniatures.
In the meantime, however, I sometimes have the impression that some boxes today are purposely bought and put back in order to be able to offer them later on at a high price on ebay.