The current system is like Churchill's description of democracy: the worst system, except for all the others.

Biology is extremely complex. There's no substitute for actually trying things out on subjects in vivo. For many diseases we don't even know the cause (Alzheimer's for example). Drug companies have all the incentive in the world to improve the system to get better odds; it's not like they want drug discovery to be such a crapshoot.

We also don't understand how some drugs work, either (e.g. Tylenol).