They don’t know them all for sure, which is why that monitoring is important. The same happened with SSRIs and other drugs that are not fully understood. After Vioxx gave thousands of people heart attacks, pharma companies now have to monitor drugs safety and efficacy even past FDA approval. I used to pull reports from the HCO I worked for so the pharma company that made the drug could measure side effects. Still, those potential side effects are often less severe than the cost of non-treatment; in this example diabetes and heart disease.
As for your second question, in my view that’s just because our society puts too much stake in wealth. It’s assumed the rich are rich because they’re better and smarter. When they get all the way up to CEO their heads have gotten so big they think they know better than scientists because obviously they’re in their position out of their own personal work ethic and not thanks in part to other factors like luck and those that supported them along the way.