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...
The reality is so much more complicated, though, that even good analogies don't provide us with any path forward in terms of actual changes to make.
In simple terms, at least.
The complicated bit comes in knowing what to change. There are some diseases in which the entire disease is caused by a single mutation and changing that one mutation in the germ line would be sufficient to correct for the disease. However, most situations involve complex interactions with many genes in a way that making a single or small simple change would have many side effects, or cause other serious problems.