What does "the lights are on but nobody's home" mean — and why is it funny?

informal, humorous

Meaning

A jokey way of saying that someone is not paying attention, or is not very bright.

Where it comes from

A twentieth-century comic comparison that pictures a person as an empty house.

Why it is funny

It turns a human being into a building. You picture a cosy, well-lit house that, when you knock, has absolutely no one inside — a perfect image of a blank, vacant stare.

Used in a sentence

"I asked her the same question three times and got nothing — the lights are on but nobody's home."