And no way to really test it without just running it in production!
To simulate an entire human body is computationally mindboggling. The number of cells are in the tens of trillions and we'd need to simulate their responses, not just to the drugs we care about, but doing so while "operating" (feeding, sleeping, etc.) the simulated body.
And at a certain point couldn't you just feed the simulation DNA and watch it grow?
Our genomes are ~1.2B to 3.5B (sexual reproduction and life at all, respectively) years old.
Now imagine trying to patch that codebase in production...