This may be because it's cheaper/better for them to ship other orders instead of yours, since they can only handle a finite amount of orders per center at a time :)
They make money off you either way, when they sit on the order for 5 days, they probably book your revenue a month before they need to pay for the product.
They buy stuff on net-30 terms, and ship stuff to you the day it arrives. WalMart does similar things for many products -- they make vendors carry inventory until it arrives at a DC.