Because the store would be 100x more crowded than its already gonna be...they would have to have armed guards at the door like a Vegas pawn shop! Apple stores are crowded enough when there isn't even a new release item,so you would have people who waited overnight for the launch and people waiting for in-store pickup, and this doesn't include all the extra people who are around just cause of hype. Where do you think all these people will fit? It just doesn't make sense to have total in-store pickup for this type of launch. Its just not safe.
and the other simple reason is the amount of stock readily available.