Apple tries to keep manufacturing cost way down since it is a smaller company than dell. Everything is pre-packaged to the most part like buying a vacuum in a electronics stores: all the accessories are packaged inside nice and neat and you don't get options. If Apple went to a Dell custom order bit, they'd loose a lot of profit margin (and Apple really, really likes its industry leading profit margin!)With Dell, since the quantity of machines sold is high, the cost of custom ordered items is dispersed over a larger product base.
(Apple's current custom ordered machines are a joke and I'd liken it to Dealer installed accessories when you buy a car, which is why that doesn't hurt their manufacturing process; also remember Apple's really stingy returns criteria where BTO are non-returnable (along with edu discounted items). It's all in place to make sure once items are sold, the money is in the bank and there will be no more expenditures to cut into profit margins.)