Because When active, Our brain produces electrical currents due to nearly 100,000 chemical reactions occurring in it per second. These chemical reactions are carried out in our brain by tiny cells called neurons. Neurons process and transmit information through electrical and chemical signals that they pass on to each other. They pass this information at the speed of 420 km/hr. Which is faster than the speed of a race car! After processing this information, when we arrive at an insight, our brain literally lights up! that is why an “idea” is usually associated with a light bulb.
A lightbulb appearing above someone's head meaning “I have a great idea” is because, A) a bright image like a lightbulb is a metaphor of a bright idea, a good and smart idea. ... So when someone has a bright idea, they're having an Edison moment, like Edison's bright idea of the bright lightbulb.
