Unfortunately, many of the decisions made by the pharmaceutical industry are based on financial considerations. As I said before, the drug approval process is a lengthy and costly one. Some drugs are extremely expensive to develop and produce, so despite our complaints about the price of pharmaceuticals, the company might not be making a profit. In some cases, a drug will make it all the way through the approval process, then when it finally starts being used in millions of animals or people, side effects are noted that take it off the shelf. The process starts with the discovery of a compound that might have a potential use as a drug.
Many drugs have very novel beginnings.
For example, many of the antibiotics come from different fungal species that produce chemicals as a natural defense in their environment. In fact, research is quite a lottery. The chances of finding a useful drug in that manner is about the same as winning the Lotto. For the billions of compounds tested, there is a relative handful of useful drugs developed. The plant world is a source for many drugs.