Why is z-index 999 not recommended for DragOverlay?

[alexeyr-ci2]

https://docs.dndkit.com/api-documentation/draggable/drag-overlay#z-index says

The zIndex prop sets the z-order of the drag overlay. The default value is 999 for compatibility reasons, but we highly recommend you use a lower value.

But it isn't clear why; and if the default wasn't mentioned, I probably would have used 999 anyway.