Because waiting in line sucks.
Apple stores are crowded places, and no matter how many people are working, it seems there just isn't enough staff to go around. However, all that could change soon, as Apple is rumored to be bringing out a new application that will allow customers to pay for accessories in the store.
MacRumors reports that Apple is planning to release an update for its retail store iOS app that will allow consumers the advantage of self-checkout for purchases of accessories and other off-the-shelf items from the store. The Apple retail store application currently allows customers to research/order Apple products for deliver; request help, check in for reservations, meet up with a Specialist or Trainer; make Genius Bar and One to One reservations and sign up for workshops and events; and reserve products for pick-up. With this new update, customers would be able to use that app to charge purchases to their iTunes account. They'll then show an emailed receipt to an Apple Store employee before leaving the shop.
The retail store app update is apparently coming November 3, which is this coming Thursday. However, Apple has not yet confirmed the news.