You would think that by now companies would be able to offer great deals on pre-built machines. It just makes sense that if they can buy the components in bulk they can sell the complete machine for less than what a user can make on their own.
The added benefit is that you create a customer, and as we all know- gamers are a very loyal group and love influencing others. Of course, there is no "loss leading" in PC hardware but I don't think its a complete impossibility.
With notebooks quickly replacing desktops at almost all levels, there is a real opportunity to converge on gaming machines and the fact that no builds their own notebook.