unless they are plain right stupid, I reckon these doctors want an easy life so that they don't have to listen to their patients' requests to investigate their parasites symptoms, most likely this is the reason they continue to deny the existence of any type parasite in a human as they have better things to do with their time in surgery and out of it.
not all but a good proportion of doctors are just about capable enough for writing out the cause of death on a certificate which has on occasions been incorrect.