Why are pharmacology studies so experimental?
The First thing is that that you can't study any process until it happens. So even if you know the structure of receptor molecule, you can't always be sure that it is completely safe.

It does seem a bit random in beginning but this is how it works. You can't just derive a medicine or drug and send it for mass production. You never know what are the side effects of the drug. For this reason, the drugs are tested on other animals (usually rats because they are quite in number, having short lifespan and not many oppose experiments on rats ethically.)

If the side effects are severe, then the drug is banned from mass production.
But if such drug is produced,more harm will be done than good. Also you can't give it to humans first. If all tests are successful, then only the drug is tested on humans.

So Screening and Testing are indirect but essential steps for studying the effects of it.