You just think that because you don’t know how the drug discovery process works.
There’s a step called “lead optimization” where human chemists literally add atoms to drug-like molecules (“lead”) and tests its various properties (toxicity, potency, permeability, …) and iterate until they find a molecule with desired properties (literally “hill-climbing”).
The whole idea of drug trials is to validate those properties in actual humans, in a way that makes it very hard for pharma companies to game the process.