Many reasons. One of them being doctors won't prescribe an off brand drug and insurance won't cover anything related to it.
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.
Utter nonsense, especially point #1. Drugs absolutely are not random mixtures. Semaglutide's molecular structure is completely characterized -- on its wikipedia page, in its patent application, and elsewhere -- and it's very easy to make identical molecules in the lab.
Novo Nordisk's patent -- US8129343B2 -- lays out the process, which is nothing more than Fmoc solid state synthesis. "The protected peptidyl resin was synthesized according to the Fmoc strategy on an Applied Biosystems 433 peptide synthesizer in 0.25 mmol or 1.0 mmol scale. . ." So they don't make it in cells, but by totally conventional peptide manufacturing techniques.
As for #2, you must be unfamiliar with the existing gray market for pharmaceuticals. Among others, both the bodybuilding community and the nootropics community experiment extensively with pharmaceuticals -- and they do drug synthesis and distribution in a way that's actually extremely cost-effective! (Necessarily, because their customers are humans who pay out of pocket directly.)
#3 is trite and overstated. It's a standard drug industry talking point, but it really has no bearing on this discussion.
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.