> Why is it certain that this is a joke and not some kind of wisdom, charm or unknown literary category?

The distinction between those may not always be that well-defined.

I once read that the story of the Good Samaritan (by Jesus) follows the structure of a popular set of jokes (or possibly self-congratulatory "wisdom") of the time: something happens, a priest comes by and is useless, a Levite passes by and is useless, and finally a common Jew comes by and fixes the problem, and everybody gets to feel good about being a commoner and not some useless elitist. Except Jesus tripped up his audience by replacing the Jew with a Samaritan.