The human body (and biology more generally) is fractal-like in its complexity. One day we may be able to model things accurately enough to not require experimental work, but probably not in our lifetime.
Now when you get into whole body or things with mutations such as cancer I agree with you while heartedly. While we can understand in a petri dish how to kill something in a human you've got to worry about things like delivery, hidden single cells, toxicity, and drug proliferation all of which can be different in every single person with a cancer. In biotech we're still just barely out of the dark ages.
To bring this to the original topic, we're still in the dark ages with fields like economics. While we understand many of levers that exist we have little idea of how and when to effectively pull them.
There are all sort of models still widely believed today. Some are unassailable (evolution, germ theory), some seem right more often than not ("don't eat bacon"), and some seem so right but nothing based on them works ("cholesterol is bad").
So this sort of naive belief in our ability to model the processes in a body or a cell has out of fashion at least since WW2. It's still used to come up with ideas to try. But even there, purely random exploration of the search space is not consistently worse.