There's no business incentive to allow a second-hand digital games market, and there's no regulation to force them to provide one. It's pretty much that simple.

NFTs won't solve this, and even if there was a mandated way to sell "used" digital games (a concept that's actually pretty bizarre when you think about it) it would not be through NFTs or block chain, because the underlying technology is slow and costs a shit ton to run. Unless you're producing a coin on the side, there's also no simple mechanism to offload the costs of running it.