You'll probably get this same response over and over again, but the reason you can't is cost and thermal characteristics. A company like Dell spends a lot of time designing notebooks around various components to make sure they don't produce too much heat so you don't fry your lap when the computer is sitting on it. They can also buy parts in HUGE bulk. This drives down prices considerably. When you haev to buy your own case with display, motherboard, graphics card... you are buying single pieces which cost a lot more. You also need to make sure everything fits in the case. It's probably not as straightforward as it looks.