Therein is part of the issue: Silicon Valley is built on software, and software development is very different from medicine. Not trying to say one is harder or easier, but one is a much more unconstrained space of creation in which a really good, revolutionary idea and a strong mission can create a company, and in the other... a broad vision might simply be impossible. "We're going to build a social network online where people can post and see their friends' posts" is a nice vision that can be implemented in code; "We're going to diagnose lots of diseases based on a single pinprick worth of blood" is a nice vision that you can't actually do.