Because in the end the pilot has the last call, he will get a ground proximity warning but the plane does not take actions by itself, for obvious reasons - the pilots are still in charge.
There are several "layers" of auto pilot, the Flight managment computer is the advanced one were u set up the entire flight with approaches etc. The "regular" auto pilot reacts only on input of speed, heading, cimb rate, altitude etc. - Simply tell it were to go and the plane goes there. Auto pilot reduces the work load for the pilots, its not replacing them. It still needs the correct input to fly correctly.