I would never build my own system not because I don't think I could build it, but because I don't have enough compatible spare parts lying around to swap things out if something goes wrong. Recently my graphics card starting acting up and I guessed it was either the memory or the graphics card, I had spares of each, so it didn't take long to isolate the problem. Now imagine having (7 or so) new items all of which have to work perfectly (more or less), before you can tell if any of them work. If you have extra parts to swap in to check if things work, no problems, but that is not everybody. Then there is BIOS configuration, Windows installation, drivers, and overclocking. I tend to pick the parts I want in my computer, someone else puts it together and hands it back to me in working order. If I want to upgrade anything, I am able to do that.
P.S. Why is that alien's hand up that girl's skirt in the picture.