I think we're going to have to give a little on the established canon. While it makes more sense to construct a huge vessel, like the Enterprise, in orbit, it limits the story. From what has been revealed, Kirk is not in Star Fleet when he sees this Constitution ship. It is very powerful moment to see a rebellious Kirk ride up and see the vessel so close. To see him dreaming of commanding such a vessel. It would be difficult for a civilian to have such a personal moment like that if it was being built in orbit. I know the first time I saw a Space Shuttle up close, it inspired me much more than seeing it in pictures and on film.