many reasons,
1). Price - DIY is cheaper
2). Custom types - if you dun need too good graphics card, or only
need 100Gb drive, you can pick and choose
3). Individual stuff also comes with guarantee and you can go back
to the shop if anything is wrong
4). if you still have the monitor and printer, you dun need to buy
whole set again like off the shelf...instead, just buy a custom DIY
desktop from SLS, and use the old monitor, printer etc...cheaper
right? 
