You're right, it's not a design decision. That degree of optimization is not something that happens at the product design level -- it is organization-wide optimization.
When Apple leverages iPhone R&D for their MacBook, they get it for free. If it takes Dell 50 million dollars to develop the tech, and Dell expects to sell 100,000 laptops, then if we assume everything else is equal, an equivalent Dell laptop would carry a premium of $500 over a MacBook. In which case, they probably wouldn't sell 100,000 laptops, and it would be a failure and a waste of 50 million bucks.
When Apple leverages iPhone R&D for their MacBook, they get it for free. If it takes Dell 50 million dollars to develop the tech, and Dell expects to sell 100,000 laptops, then if we assume everything else is equal, an equivalent Dell laptop would carry a premium of $500 over a MacBook. In which case, they probably wouldn't sell 100,000 laptops, and it would be a failure and a waste of 50 million bucks.
Horizontal integration is powerful.