Nor did I say that it can't be. but to even suggest that this is the reason for the dismissal is assinine.
A joke is a short story or ironic depiction of a situation communicated with the intent of being humorous. These jokes will normally have a punchline that will end the sentence to make it humorous. A joke can also be a single phrase or statement that employs sarcasm. The word joke can also be used as a slang term for a person or thing which is not taken seriously by others in general. A practical joke or prank differs from a spoken one in that the major component of the humour is physical rather than verbal (for example placing salt in the sugar bowl).
Jokes are typically for the entertainment of friends and onlookers. The desired response is generally laughter; when this does not happen the joke is said to have "fallen flat".