It's opportunity cost and simple capitalism. People want different, special, unique computers, except they want them with performance. So people pay more and subsequently get a computer that not many other people can get. Sure, you could buy a hydraulic case and throw your own components in it, and then cut out a few blowholes and paint it, but most people are lazy and have more money than brains, and would rather pay for a computer that has the shi0t already done to it.