At first I thought this question was related to Universities contributing in the launch of startups. In that case these are called Spin-Offs and there a few examples of very nice projects like the Lycos, OntoPrise etc.
Universities are a great place where people can discover who they want to be.
A university is like an environment where you either behave by its rules and once you learned how to get by, you can aim for being a good student and the best of grades... You don't neccesserily need to be the best in that environment to be on top of it, since it is easy to get good grades in most Universities once you know the tricks of the so called teaching and learning.
There is also the other case which I believe relates to your question: people understand that a University does not neccesarily reward that most skillfulled ones and then people decide for a different reward system and launch their own startups (drop-off, or just pass without being grade-greedy).
I am sure there are many cases of drop-offs, or people that put their PhDs on pause and decide to invest in advancing themself in science through their own companies & research.