No one is surprised that our brains are in a protective helmet -- the skull. An organism that did not provide such protection for its central nervous system would not survive at the same rate as one that did. So it if hardly surprising that the most important sections of our genetic library are protected from mutation.

The protection is, however, only to the highest degree feasible. There are limits to the thickness of our skulls as well, which is why we wear helmets in risking environments!