The comparison between startups and PhDs is not esp. appropriate. The entrepreneur often is willing to assume greater risk for greater pecuniary reward, whereas the PhD often is not assuming any greater risk but willing to accept likely lesser pay to do what he loves.

Which isn't to say that there is a class of entrepreneurs that start companies simply because they love starting companies, but that this isn't unique to entrepreneurs (e.g. there are lawyers who love doing lawyerly things).