Our own modern, Western, zig-zag lightning bolt design doesn't really look like real lightning (a naturally occurring electro-static discharge) either. We use it by convention. It is
part of our culture. We drawn it on pictures in primary school because we see examples of that
style and we keep on using it.
Maybe ours looks a
bit more like the real thing though we now have the advantage of photography that and ancients didn't.
I am not sure the ancient design ever transformed from a more natural design to this one.
It would be interesting to see a pre-Zeus depiction of lightning.....
A as last aside, maybe the Greco-Roman device is meant to convey the look and the sound - though that is nothing but a wild guess.
SC