While building one yourself is going to normally be on par or a bit cheaper, and give you more control over your system specs. Many people are just not willing to try or just fear messing up.
Printable View
There's some shops that assemble and test the system for you for ~100 bucks. And it will still be cheaper than buying a Sony, Dell, etc. system. The problem is that there's lots of components and the chance of DoA is pretty high with pc hardware. Not everyone has the nerve to debug a broken PSU ^^ The assembly itself is easy.
There is where you go to a hardware forum to ask around, in my country there is such a place, and everyday ppl ask and got their question answer, which later on very happy with their setup, the best part is that they learn and soon to be able to help more people :)
Th3 i|\|73rn3t is a source of knowledge. :3 I've learn a lot thanks to the hardware forums i was always lurking and asking :D