The reason is next to no company/investor wants to fund double blind clinical research on a natural product to come up with an FDA/medically sound treatment regimen due to there being no monopoly privileges conferred for doing so, therefore rendering it unprofitable from a capitalist free market point of view.

Big difference. It's not that all medicinal plants can be worse than the condition off the bat, it's that no one can make money by figuring out a dosage and putting it through the proper channels to allow manufacture and utilization for that purpose.

Herbalists have been doing their best to maintain a corpus of knowledge around these procedures for centuries. It isn't their fault that our boner for letting the market arbitrate ever aspect of our lives keeps useful research from being done.