Earlier quoted context omitted.
What you are talking about makes absolutely no sense. There are completely different chipsets, ports, batteries etc in a MacBook Air versus a MacBook Pro. Most of which have dependencies on each other. So a user can't just spec out a machine with the exception of very specific components. Hence why they are branded differently.
Not to mention it costs a lot to offer configurability, rather than just ordering countless hordes of a few models from the factories...
That is one of Apple's greatest strengths, and having that strength absolutely allows them to offer configurability.