Because there's a million and one things that could go wrong? Yes, we can make simulations. But the simulations are based on assumptions about the physical world. When the physical world doesn't obey those assumptions, things start to deviate from the simulation. Plus, simulations aren't perfect. We can't simulate every atom of the rocket and every atom of the atmosphere, so we have to approximate things. Errors creep in.

There is more to engineering than understanding the fundamental physics.