Question

Why potato is a stem instead of being a root ?

It's a modification of stem but appropriate reasons are required ..

Answer 1

Even though the part of the potato we eat grows in the ground, it is not the root of the plant. It is the underground part of the stem that has thickened. This part of the stem is called a tuber.

Tubers thicken and grow big because they store the food supply for the leafy part of the plant above ground