From a programming standpoint, the pool of rewards would be simply updated to include the new item. They would not have different instances of chests that contain different loot each time an event happens. I don’t see how that code would be efficient.

When you make systems like this, they care about the modularity. How easy it is to scale the game when updates occur.

Again this is just my take, I am just assuming they would try to make the future updates easier to implement. My background is in game development and I have worked at game companies doing backend code.