A lot of Chuck Norris jokes are funny.He got the reputation as the man that can do anything,because he doesn't wear protection and he hasn't got a stunt dubbler,also he's famous because he beaten up Bruce Lee.
Why do people do this? It's self-explanatory really.
On a less serious note I do believe the first Chuck Norris joke was by Conan O'Brian (mispelling?) on his old TV show before he took over for Jay(J?) Lenno(?) and after that it took off.