Bright flashes and jagged bolts of light doesn't necessarily lead one to intuitively believe "Oh, look, there's a source of energy in the sky which can somehow be generated and harnessed to power machines and light bulbs."

We think that way because we have the scientific knowledge of what it is and how it works, but that's all retrospective. Prior to the discovery of electricity as a concept, lightning simply appeared to be some divine weapon wielded by angry gods. Even atheists of the early-Enlightenment era wouldn't have understood it.

That's like saying "Humans have observed fire since prehistoric times, so they must have understood chemical bonds and exothermic reactions." It simply doesn't apply.