Because "Patient" impedes the flow of the statement.
Look, there is nothing wrong with using the first name of the patient. If it bothers people that much, just use a fake first name and put it in quotes. At least it won't jar you out of your read by seeing the word 'patient' as the name.
"So I asked him: "Mr. Patient, how long have you been feeling this way?"
"So I asked him: "'John', how long have you been feeling this way?"
That's the problem with narrative PS. You want to draw the reader in and capture their attention. Anything that jars them back - like using 'patient' as the name, is a negative.
Christ.