Because there are too many different variations?
Here's the one I always think of first:
There once was a man from Nantucket
Whose dick was so long he could suck it
With cum on his chin
He said with a grin
If my nose were a cunt I would fuck it
Heh heh... the SCAdians I hang out with on occassion have an annual event of silliness, including a limerick contest. I've won both times I've managed to make it, and last time I won a dagger in the process (it's +3 against men from Nantucket!
Speaking of which, pictures from the new years party said SCAdians threw that I went to are finally online, just thought I'd share one of the funniest bits of atmosphere, a little Dr. Suess parody (see, this is vaguely related - it's poetry).