Flight crews used to have to see a physician before starting their duties (it may even still be that way in some countries).
Here is a good reason to implement that: to provide air crew with a safe way out. If it's the doctor that's declaring you unfit to fly, you don't have to make that dreaded call and explain yourself. The reasons for being unfit to fly are medical and thus privileged. Is it a cold, fatigue or alcohol? No need for dispatch to know.
Of course, recurring alcoholic incidents will be reason for a deeper investigation, but that will be triggered from the medical side through a suspension of a medical certificate. Which could be done for high blood pressure as well. Again, a face-saving way out for the air crew.