[1] Scroll down to "PEOPLE": http://paulgraham.com/vwplan.html
The "I need X, but existing X is nonexistent or crappy, so I'm going to make my own X" thinking seems to be a common motivator. Technically-competent entrepreneurs often fall into this case.
Meet some more grad students and you'll get a clearer picture of this fairly quickly. This is at least a consideration for many engineering grads.
You won't find too many PhDs starting companies that make web apps and iPhone games, though. They're starting companies in the really high tech areas, areas at the forefront of technology that folks without PhDs don't understand just yet.
Edit: Or they wanted to start a business in a field where graduate level credentials are required for people to take you seriously (e.g., biotech, clean energy, medical devices, etc.)