[qsera]

The reason is that we cannot fully figure how evolved systems work. Let me explain..

Let x be the set of all physical behaviors in nature. At any point in time, we (human beings) are aware of a subset of x, let us call it y.

When a human entity design a system, they can only use the behaviors in y. But an evolved system could potentially use all of x. Since y is always proper subset of x, this means that we will never be able to know everything required to safely modify an evolved system.