[dunmalg]

A) I think this is one of those jokes that has enough of a core of truth that it isn't entirely a joke. Murphy's Law is likewise a joke, but it speaks to a core facet of reality, so it also gets treated seriously.

And B) there's the fact that Greenspun's 10th Rule is written using just enough technical jargon that the vast majority of non-programmers don't see the joke, and therefore take it as a serious assertion. If he had said "C/Fortran programmers keep reinventing the wheel, only square shaped and crooked, while the round wheel Common Lisp already exists", they'd delete it as a joke.