Why are potato ginger and onion not considered as roots?

Potato, ginger, and onion, unlike carrot and radish, are not considered as roots. This is because, they are not derived from the primary root system of the plant.

Potato is a modified stem, more precisely, a stolon. It develops from the underground modified stem called rhizomes.

Ginger is also a modified rhizome.

Onion is more like a modified leaf base.