Because if everything was private you wouldn't be able to access those functions. Only the class functions can use private functions. Since you change all the public functions to private there is no public function that you can access to use those private functions. Its basically a useless class since everything is hidden.
I was just curious what would happen if you have a working program with several classes and you decided to change all the "private" access specifiers to "public." Would the program run the same way? How about changing all "publics" to "private"?