Back in 07 when i bought this computer from Dell, I really wanted to build my own. I had even priced what I wanted and it would cost me around $800. Sadly, the pc I was using at the time was having random hardware failures, and as I had already spent $200 replacing bits of a 5yr old box, I figured it was time for a whole new box.
The thing was, Dell extended me $1500 in credit. So I could have waited for months, buying a piece here and there as I could afford it, or I could buy a pre-built from Dell and get it right then.
I think that is the reason most people buy the pre-built systems, they want a whole system that works out of the box, right then. And a lot of the major manufacturers offer lines of credit.