When I worked for Apple they didn't have this process and I *hated* selling iPhones.
You never knew how long the transaction would take. Some customers had all of their information in order and it was pretty quick to sell them an iPhone, but it was pretty common to run into a customer who had some problem with their wireless account that turned the process into a 45+ minute ordeal.
So instead of tying up an employee for 45 minutes because someone has a past-due amount on their account, or wants to upgrade when their carrier says they're not eligible, or wants to upgrade but doesn't know the SSN of the primary line (or put extra security on their account and doesn't remember the code), or thinks they can do an upgrade swap with another line on their account but doesn't really understand how that works ... let the customer spend their time figuring that mess out while placing an order on a computer in the store without having to tie up an employee?!
Hats off to the folks in this thread who have everything in order to do a quick "I want to walk-in off the street and buy an iPhone" transaction. Unfortunately not everyone walking in off the street is as organized as ya'll and it used to result in tying up employees for a long time.