Not that simple, as someone else already said, the different departments are very different from each other, the Samsung Galaxy people would have to order from the component people the displays for the phones. They act very independently of each other, in a way they are basically independent companies, just operate under one umbrella.
Apple probably has companies bid to be component providers for Apple products, where they have to bid against other companies for "exclusive" rights. In the case of the retina display, both Samsung and Sharp, who have the factories and ability to build the displays to Apple's specifications of course, "won" the bidding and provide the displays for Apple's products. In return for the huge amount of business they get from Apple, it also has other stipulations, like probably their stock of the retina display at that size and/or density is completely taken by Apple until mid/late this year due to the high volumes ordered (even if the iPad doesn't sell to projections somehow, deals such as this example still happens as a business move in many cases), so they couldn't even put it in their own products until then if they wanted to.
