Humans are incredibly complex systems (and systems of systems) and our knowledge of how these systems work and interact is very limited despite the amount of work put into improving our understanding of them.

We can try to simulate the interaction between a drug and a protein, but most of the time the data to say what the downstream effects (if any) are simply does not exist. This is further complicated by the fact that these downstream effects can be extremely varied depending on both the tissue/disease you are targeting and the person you are treating.

Of course we are always generating more data from more diverse populations and more conditions, but there is a very very long way to being able to simulate drug trials like this.

We know so little about biology that the only way to do it is test on humans.

In another comment I said that if the area of biology/microbiology etc were the size of the surface of our sun, the area of our knowledge in those subjects would be perhaps a square millimeter.

And if you think that’s bad, don’t get me started on higher level areas like neurology.

nowhere near accurate enough, i would estimate we would need 2-3 more orders of magnitude of data to make something remotely useful - i'm not a biologist though