Because not every medical condition warrants for immediate landing. For example, if a pregnant woman is in labor in air, unless there is any complication, it is best to have the baby delivered first.

Airlines have access to professional medical advice, as well as its flight crews are trained for first-aid. So they are trained to handle issues like this.

In term of the paramedic, in the best case scenario, it will take a plane about 5 minutes to land and the paramedic can be met. Those 5 minutes could mean life or death to a person. On that basis, paramedic is secondary compare to those who can offer assistance.