If we're talking about a small store with little storage space like GameStop, they probably are quicker to rotate old stock away - that could be via donation or return to corporate or manufacturer. Maybe some stores have gotten better tax breaks for donating something versus tossing it. Bigger stores with large stock rooms in the back occasionally hold secrets, waiting for a time when the manager decides to deal with it. I didn't get the inside story on how old stock shows up on shelves again in every case, but I can make some guesses. I bought a NOS Astrocade from a member here, years ago, which I believe was stock at a mom-and-pop store that didn't sell. One out-of-the-way Target had a new copy of Metal Gear Solid 3 on sale about 10 years after the game release. A Walmart had a bargain bin of 5-7 years old PC games show up one random day in the early 2000s. A Best Buy had a bargain bin of over ten year old PC classics like Tomb Raider 2, 3. My old local Goodwill got all kinds of old stock which had been discounted repeatedly at its original location and remained unsold. These days I've seen a fair amount of early Xbox One / PS4-era stock at many stores, and quite a few Xbox 360 games too.

Remember the difference between overall rarity, and rarity of a certain condition. Supply and demand doesn't just apply to a title, but also to all the various grades. Lots of things aren't rare but they also weren't kept in good shape, so anything that survives in factory minty shape can be worth a substantial premium if demand is there. You can see this at work in markets as different as cars and baseball cards. The reference books for other hobbies have different prices for different grades and weird things can happen (i.e., the widely-hoarded 1950-D Jefferson Nickel).

It sure it true that you can expect most media to last a long time, but it's not a guarantee. That's why I wrote earlier about sealed games being just an illusion of preservation. Unless what you're interested in is the "factory scent" that might still be lingering inside (mmmm, volatile organic compounds!), you could do a lot of preservation work for the money rich lazy yobs throw at slabbed games. It's a different kind of thing than a coin or card where a slab is one of the best ways to preserve condition and value while still being able to enjoy the item. But that being said, I've got a fair number of sealed games because I haven't been willing to lose the "value" or deal with the work of preservation, so I shouldn't throw too many stones...