Belief systems tend to be very deeply entrenched.
The burning of heretics, the Inquisition, torture chambers, etc. did not shake the faith of numerous Christians in the Middle Ages, and how far off the beam can anyone get? How thoroughly can anyone pervert the teachings of Jesus?
Some of history's bloodiest battles were fought to bring Europe out of this darkness.
The atrocities committed by Muslims today shock the world, but they may not be sufficient to shake the faith of the faithful.
...Or those who do leave Islam, have to keep it quiet becaus to leave Islam makes one apostate and subjected to death for leaving.