As I understand it, the problem with drugs that aren't patentable is there is no way for those who pay for clinical testing to re-coup their investment. Phase I-IV clinical testing usually goes well over 100 million USD [1] so drug companies need to recoup that money, if the drug isn't protected under patent, once approved, any manufacturer can produce it. Sure they could patent GMO organism that produces it, but the process to produce it chemically is already well understood, Shulgin has published recipes to produce virtually every psychedelic chemically pure in Pikal and Tikal.

[1]https://aspe.hhs.gov/report/examination-clinical-trial-costs...