When shooting any animal your first priority must be to ensure a quick and humane kill and in deed a well-placed head shot will do this. The problem is that a deer’s head is a much smaller and more mobile target so it is more difficult to hit in the first place. What is more a poorly placed head shot, such as a jaw shot, will result in a wound that will lead to a long and painful death with very little chance of finding that deer, even with a trained tracking dog.
The chest area of a deer is a much larger target allowing greater room for error and any deer shot in the body area can usually be found and dispatched humanely if required. I have seen far too many experienced stalkers “miss” head shots to think that this a humane alternative except in very specific circumstances.