^Rong
It all boils down to cost, most large companies lease their computers. Dell has excellent corporate lease/service package that make them a desireable choice. When making a computer choice for my workplace (yes, I'm a beancounting fuck), Dell gave us the best ROI.
Now I know that you all nub your hand built hotrods, and they are awesome and shit, but sometimes the factory option makes a lot of sense. I recently bought a Gateway, it fucking rocks, it has a shiny butan and everything!!