There are some good problems that have been asked over and over again because they teach good lessons. My data structures professor started one of our assignments off with the following quote "More time has been spent on undergraduates recreating the Ackerman function than any other problem in computer science, and you all will be no different"
Sure there are other problems that have double recursion but why try to find something new and different when a good problem already exists? Plus there is something unifying about it. If I meet someone who graduated years before me or years after and they also had to do the Ackerman function in some language maybe the same one I used it kind of give you something in common. I like that; a common thread the ties us all together.