[aeorgnoieang]

I'm sure there are other barriers as, if what you are describing is complete, one could earn a pretty riskless profit by some combination of (a) 'threatening' to become a new entrant in the market for an expensive drug; (b) stockpiling the existing entrants drugs when they drop their price to try to drive you out of business.

I remember reading a book about the history of some (famous) chemical company in the U.S. – Dow? Dupont? – doing exactly what I described to compete with the at-the-time existing German chemical company cartel.

[Symmetry]

For more regulated classes of drug the FDA doesn't just control the conditions under which it is made but also assigns production quotas to different manufacturers. The prime worry, I believe, is that if too much of a controlled substance is produced and it isn't sold then it might end up being sold illegally. There are also issues where allowing people to produce a drug willy-nilly would stretch the FDA's inspection capacity and allow the possibility of defective drugs being made.

Plus there's the issue of whether a drug can remain safe to use after long periods of time at all.