Technically, they are being grown by stacking them, a lot of things grow vertically as opposed to horizontally across the ground.

Some potatoes are indeterminate. Which means, as they start growing you can put more dirt on top of them so that they produce more potatoes as they keep growing up. By growing potatoes in a container, you can start with just a shallow amount of dirt and then add more as the plants grow.

Determinate potatoes will only produce at the root area.