Genetics is oversimplified to non-physicians. It's cool that we can diagnose and predict the likelihood of getting Huntington's disease using our knowledge of genetics, but extremely few diseases are this simple. There are huge swaths of the human genome that we don't understand but are likely playing some important role in the regulation of other genes and diseases. We are nowhere close to being able to look at a patient's genome to predict anything useful outside of a handful of exceptions.
Patient histories are honestly often garbage—I say that as a physician. I look through dozens of patients' charts every day, and there are constantly errors, incomplete documentation, and fragmented records across multiple institutions. Just last week I read a chart for a patient who had a documented hysterectomy from years ago. The brand new CT scan I saw showed a perfectly normal uterus. Once something goes in a patient's history it's nearly impossible to correct or remove. If some doctor from ages ago said the patient is allergic to medication X, but the patient denies it, what do I do? Usually, we opt to leave the allergy listed out of fear of the consequences if the patient is wrong.