I suspect from the collective psyche of the human race, responding to the need for a safety valve to drain off the tragedy, silliness, chaos, and so on we see around us. Don't forget that there is a whole subgenre of humor which does nothing but help people cope: combat jokes, cop jokes, ER jokes.... These are the ones that are too, too "rough" for general dissemination but are hilarious -- and deflecting -- to those who share the ambience (e.g., using a skeleton with a cigarette in its mouth to point the way to a combat unit's headquarters, as was done in the trenches of WWI).

Humans seem to need their sense of the ridiculous, whether it punctures the pompous or points up the superiority of the peasant to the ruler.