you will never stop people buying from the big PC manufacturers because like has been said its just easier that way.
But look at it this way, if it gets more people into computing then its got to be good right? My first PC was a pre-built from a certain large company. But as I got more into using it, surfing the internet, playing games etc; I soon started to realise I wanted more performance out of the PC, so read up on how to install components and which ones to choose, and went on from there.
And here I am now, Ive built countless systems from scratch, modded them, enjoyed using them, sold them, and, oh yeah - destroyed them