Reread this thread. On the first page, Peter Elliott explains how to check your digital orders and see if a gift has been redeemed. If it hasn't, you can get in touch with Amazon Customer Service (*not* KDP) and see if you can get the money back.
That's the Amazon Gift Card Policy. It states very clearly that gift cards issued after October 2005 have no expiration date, and cards issued before that date in 13 US states also do not expire even though they came with an expiration date. Any other cards that were purchased before October '05 expired on whatever date was indicated at the time of purchase. (My guess is that the reason 13 states are an exception to that last part has to do with laws in the individual states.) Since the gift cards don't expire (with the exception of some that were purchased a dozen years ago), the answer is NO Amazon doesn't pocket the money, people can still use them.
I posted two responses to the original poster of this antique thread. The second one explains how to check the status of any gift card you purchased. You can scroll up and find it. It was posted on Mar 14, 2014 9:51 PM.