i bought a desktop back in 05 from cyberpowerpc, because it was about 100 dollars cheaper than buying all the parts separately on newegg. (sometimes it works that way, most of the time it's cheaper building your own.)
for expensive hardware such as the 780gtx, the savings gap tends to widen in the favor of building it yourself. for hardware in the mid-range/low-range, tends to be the sweet spot for companies like this.
i can't wait for haswell to be released. coupled with the 780gtx, minimum FPS at 60, awww yeah. smooth as butter!