Because HE is the one who knocks.
They always talk about how they're from New York.
You tell your Wife, "I saw a lady, looked exactly like you" Wife asks, "WAS SHE BEAUTIFUL?" You cant say 'NO' You cant say 'YES' That is Checkmate!
A bird who knocks before delivering its message !
Someone who knocks on your door for no apparent reason.
Because it was on the house.
Shaking Bad.
Knock-Knock Jokes
Nobody asks, 'who's there ' when you try and tell a knock knock joke.