I understand that the gas black needs to be in a certain place, but why is the front sight always on the gas black?
The front sight isn't always on the gas block but often is. The gas block has to be securely mounted to the barrel with no shifting or cant which would allow a gas leak or stoppage. The front sight also has to be securely mounted to the barrel with no shifting or cant which would knock off its mechanical zero. So it made sense to incorporate both systems into the front sight base.