Isn’t that incredibly wasteful in terms of space and materials? Why not just use a few large lithium ion cells?
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So the batteries are made with what are essentially layers of battery chemicals that are rolled into a tube. With a prismatic (rectangle) battery you still have to roll them, just in a rectangle shape, or stack multiple independent sheets that need connected together with tabs. This adds corners/edges that are nonuniform and lead to a higher rate of manufacturing defects and sensitivity to deformation. Especially when in a big block instead of a narrow cylinder.
So basically round is cheaper and easier to develop for high current, high capacity, reliability, resistance to deformation, etc. And 18650s just a really convenient size for general purpose so it’s had a lot of manufacturing tailored around it.
Note that lots of EV batteries use prismatic (blocky) batteries. But Tesla which uses the cylinders has a big focus on fast charge, fast discharge. And for that you need high quality, lots of surface area, and watercooling.
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