Why are lithium ion cells mostly round?
1) The cylindrical shape is very easy to manufacture - You basically fold the layers around a round object.

2) For the same reason as drinks cans - this shape is something between the Sphere (high strength and damage resistant) and Cuboid (Easy to stack and low space is wasted). It is the best trade-off between strength and volume.

3) This way You can easily stack the batteries - (Parallel and Series connections).