I wonder why...

[Foxster]

...I can be landing on a moon or planet with a huge bright rocket engine firing away and yet everything is pitch dark but if I switch on a single small lamp half the planet is lit up?

[cantab]

Because Squad didn't think of making the rocket exhaust work as a light.

[WOODY01]

That happens sometimes. When I first noticed it I did not know it was a glitch and tried to launch satellites so the entire surface of Kerbin would be illuminated constantly. I learned after the getting the second sat positioned it does not happen all the time.

[magnemoe]

Because Squad didn't think of making the rocket exhaust work as a light.

Also at it would cause lag issues then using large rockets with lots of booster on an night launch.

Light have limited range now, remember back in 0.18 we could light up continents

Light is very nice then landing anyway even in daylight as it give an distance indication.