I would wager that most drugs are discovered by startups. Pharma has a very well established "farm team" system where venture-backed companies do the compound discovery and complete Phase I and perhaps Phase II trials, then get bought by one of the major players, for exactly the reason you mention -- nobody wants to rebuild the infrastructure to manufacture and sell pharmaceuticals.

However, the founders of those startups are rewarded handsomely for their effort, assuming they are successful.