I think it is a matter if you want to actually want to do the building or not. Some people get nervous working on expensive equipment and maintaining it and troubleshooting it if there is a problem.
I love building computers even if I could get it premade for the same price I would still build it myself. You would have a lot better control over the components that are in the machine and also if you can upgrade down the road or not. And it is a good learning experience. Sometimes it is cheaper sometimes it isn't (usually is though)