There's no reason to believe that's true, that there some inherent reason we can't change our selves to better fit our new environment. Arguably that's exactly what humans have been doing now for tens of thousands of years.
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.