Dunno. Probably the same reason some people now seem to like to joke about Chuck Norris. We (as in all Americans who make said jests) don't know, it's just fun to do. Though I'm willing to bet some people do it out of genuine dislike for the French. :D And just so you know, I don't make jokes about France often. I just decided to use this forum as an opportunity to post some bad France jokes.
BTW, did you know that there's no such thing as tornados, Chuck Norris just doesn't like trailer parks?