That is because for an obstetrician the consequences of medical malpractice are profound and expensive. They are basically either killing a child or crippling that child for the entirety of their life. If the child survives their malpractice then that child will have a life that is much shorter, or much more unpleasant, than it needed to be if the obstetrician did their job correctly and the insurance company will be paying out big money to support that child for the entirety of that child's life.