There is no legal obligation on a doctor to assist on an aeroplane, anymore than their next door neighbour.
In the past I have discussed this with a doctor friend of mine. He said that he always does repond if such a call is made but he know of many doctors who do not.
Some, when travelling, ensure that 'Dr' is NOT on their tickets and will not 'make themselves know to the staff'.
That is their free choice. NOW should we move it to Jet Blast??