The inability to change their products is directly tied to the restrictions in the problem space that let them make the defaults great.  You're right, you can't change them.  Which means you can't screw it up.  Most people are not tech-savy enough to handle the choices, so instead of giving people something mediocre and saying it can be customized, Apple makes something great that is locked down.  For most people, that's perfect.  For those it isn't good enough for, they can get something else.