The other reasons are:
1/ Even the manufacturer doesn't know everything about how the drug was made. They characterize the final compound but the compound was made by some sort of living cells so each one has different properties. For the same molecule, one cell line may produce a drug with entirely different toxicology profile compared to another. You need a full manufacturing chain with different departments analyzing it at different stages to guarantee the desired effect.
2/ Patents make sure that the vanishingly few groups with the capability to do the above characterization would not want to risk their business doing it.
3/ For every one molecule that made it to market, dozens or hundreds of others were discarded. They all cost about the same amount to develop up to the point they were thrown away. Due to the amount of regulations in place, the cost to research is obscene and the final manufacturing cost is peanut compared to the price of the pipeline what the drug went through.