They have actually...
All sprites belong to a group (even if it is the default group).
The z order is the group index (ie: first sprite in the group get rendered first, and so on).
You cannot change that to a arbitrary value, since sprites cannot share a slot, neither there can be "holes" on the group array.
But there are the functions sprite.toFront() and sprite.toBack()
I personally create more groups to control ordering, one of my apps is a snowglobe, I've create three groups, one for the background, one for the foreground, and between them a layer with the snow, this way I don't have to do background.toBack() and foreground.toFront() every frame.