Usually, you can get better deals on components buying a whole bike as a package than buying the pieces and putting (or having it put) together.
If you don't know what components you need, there's not a lot of motivation to piece a bike together.
I've got friends that build up their own bikes, but they scavenge Ebay for used parts continually to get the best deals there and know exactly what they want. Something I can't very well do.