system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes clothes that are kept for too long without use develop that distinct "stored" odor?

That distinct scent on clothes that haven't been worn in a couple of years or more.

Same with fabrics on homes or cars, such as curtains and carpets.
gpt
For clothing, it's dust mites and bacteria.  Want a pro tip from someone who does costumes for a local theatre group?  If you are storing something for the long term, get a spray bottle and fill it with cheap vodka.  Then spray everything you plan to store.  Don't "soak" it, just a misting spray.  Keeps the odor at bay, and doesn't reek like mothballs.