When I first started selling games, I was starting to get nervous because I was afraid nobody was going to buy any of them; luckily, that wasn't the case, and they've sold fairly well.
Many people have a lot of pleasant childhood memories with these video games. They buy it because of nostalgia, or they simply collect them.
There are also people, who have not played them so far, and they would simply like to have the original physical copy of the game either to play, or simply to also collect them.
And there are those, who buy them as investments.
Especially the unboxed versions. Those generally are more valuable (or at least this is how many people see it), and their value is only increasing with time.
The same is the case with many hardware components too. For example PC GPUs (video cards).
And of course some of the collectors maybe are also collecting them as investments, but not necessarily.
There are many kinds of buyers.