In some cases people have been given the opportunity to buy the chips from casino some time after it closed.
Sometimes only chips in unused condition are offered, or the buyer chooses to take those.
Sometimes a large quantity of unused chips exist because they had a large reserve, or only took delivery a short while before closing.
Sometimes after closing they are dumped rather than destroyed and someone finds them.
Sometimes after closing the owner takes them home and sells them later.
Many reasons.